PERF: significant speedups in tz-aware operations
Operations involving tz-aware data currently incur a pretty substantial penalty:
```
[ 93.04%] ··· timeseries.DatetimeAccessor.time_dt_accessor_year                                                                                                           ok
[ 93.04%] ··· ============ =============
                   t
              ------------ -------------
                  None      2.41±0.07ms
             **US/Eastern     150±2ms**
                  UTC       2.64±0.07ms
                tzutc()     2.70±0.06ms
              ============ =============

[ 93.19%] ··· timeseries.DatetimeIndex.time_add_timedelta                                                                                                                 ok
[ 93.19%] ··· ============ ============
               index_type
              ------------ ------------
                  dst          n/a
                repeated       n/a
              **tz_aware     305±7ms**
                tz_naive    3.38±0.2ms
              ============ ============
```

This PR improves the performance of tz-aware operations to near that of tz-naive ones through a couple approaches:
- Eliminate a duplicative validation check by setting `_freq` directly
  - `freq.setter` calls a validation check that compares the input value against `to_offset(self.inferred_freq)`, which is exactly what we are passing
  - For tz-aware data, `.inferred_freq` requires converting the entire array to the appropriate tz. The time to do so dominates the runtime for our benchmark. Simply eliminating 1 of 2 calls cuts runtime by 50%.
- Improve the performance of `pandas._libs.tslibs.conversion` functions by batching `searchsorted` calls rather than doing piecewise.
  - In theory, both approaches should be `O(n log(k))`. However, call overhead appears to be substantially larger than the actual search time for the `N` of our existing benchmark.
  - The existing comment suggests operating piecewise is beneficial in the presence of lots of `iNaT`s, but this ignores the fact that `searchsorted` has some optimizations for when the `needle` array is also sorted (which allows it to incrementally shrink the search region).
- Minor speedup due to minimizing `is_tzlocal()`

Here's the same comparison with the PR:
```
[ 26.38%] ··· timeseries.DatetimeAccessor.time_dt_accessor_year                                                                                                           ok
[ 26.38%] ··· ============ =============
                   t
              ------------ -------------
                  None      2.31±0.06ms
               US/Eastern   3.70±0.08ms
                  UTC       2.49±0.03ms
                tzutc()      2.43±0.1ms
              ============ =============

[ 26.52%] ··· timeseries.DatetimeIndex.time_add_timedelta                                                                                                                 ok
[ 26.52%] ··· ============ =============
               index_type
              ------------ -------------
                  dst           n/a
                repeated        n/a
                tz_aware     4.01±0.2ms
                tz_naive    2.33±0.04ms
              ============ =============
```

And `asv` output:
```
$ asv compare upstream/master HEAD -s --sort ratio --only-changed
       before           after         ratio
     [02a97c0a]       [5a5ed18b]
     <tz_aware_op_speedup~2>       <tz_aware_op_speedup>
-      3.38±0.2ms      2.33±0.04ms     0.69  timeseries.DatetimeIndex.time_add_timedelta('tz_naive')
-      21.0±0.2ms       14.4±0.3ms     0.68  inference.DateInferOps.time_timedelta_plus_datetime
-        700±80ns         366±20ns     0.52  timestamp.TimestampProperties.time_dayofweek(<UTC>, 'B')
-         181±2ms       30.6±0.7ms     0.17  timeseries.DatetimeAccessor.time_dt_accessor_time('US/Eastern')
-        180±10ms       29.1±0.9ms     0.16  timeseries.DatetimeAccessor.time_dt_accessor_date('US/Eastern')
-         178±3ms       26.7±0.6ms     0.15  timeseries.DatetimeAccessor.time_dt_accessor_day_name('US/Eastern')
-         179±2ms       26.3±0.4ms     0.15  timeseries.DatetimeIndex.time_to_time('tz_aware')
-         172±4ms       24.8±0.7ms     0.14  timeseries.DatetimeAccessor.time_dt_accessor_month_name('US/Eastern')
-         175±5ms       24.1±0.9ms     0.14  timeseries.DatetimeIndex.time_to_date('tz_aware')
-         150±2ms      3.70±0.08ms     0.02  timeseries.DatetimeAccessor.time_dt_accessor_year('US/Eastern')
-        95.3±5ms      2.02±0.07ms     0.02  indexing.NonNumericSeriesIndexing.time_getitem_label_slice('datetime', 'nonunique_monotonic_inc')
-         149±2ms      2.85±0.02ms     0.02  timeseries.DatetimeIndex.time_timeseries_is_month_start('tz_aware')
-         305±7ms       4.01±0.2ms     0.01  timeseries.DatetimeIndex.time_add_timedelta('tz_aware')
-        95.3±3ms          356±9μs     0.00  indexing.NonNumericSeriesIndexing.time_get_value('datetime', 'nonunique_monotonic_inc')
       before           after         ratio
     [02a97c0a]       [5a5ed18b]
     <tz_aware_op_speedup~2>       <tz_aware_op_speedup>
+         323±6ms          386±9ms     1.19  timeseries.ToDatetimeISO8601.time_iso8601_tz_spaceformat
+         165±2ms          191±4ms     1.15  timeseries.ToDatetimeCache.time_dup_string_tzoffset_dates(False)
+         151±5μs          170±2μs     1.13  indexing.NonNumericSeriesIndexing.time_getitem_scalar('datetime', 'nonunique_monotonic_inc')
```

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=h1) Report
> Merging [#24491](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/02a97c0a150377fab02fa22580d232c8242b5baa?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24491/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24491   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         166      166           
  Lines       52412    52412           
=======================================
  Hits        48382    48382           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.73% <100%> (ø)` | :arrow_up: |
| #single | `43.05% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `87.68% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `97.74% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=footer). Last update [02a97c0...6ffad7c](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=h1) Report
> Merging [#24491](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62506ca3c37b6b0d84833a8329e50c000316250e?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24491/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24491   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52478    52478           
=======================================
  Hits        48483    48483           
  Misses       3995     3995
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (ø)` | :arrow_up: |
| #single | `43.04% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `87.99% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.01% <100%> (ø)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.49% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/accessors.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hY2Nlc3NvcnMucHk=) | `91% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.22% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=footer). Last update [62506ca...360415c](https://codecov.io/gh/pandas-dev/pandas/pull/24491?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Overall LGTM. Could you also add `tzlocal` in the relevant benchmarks `that parameterize over timezones? There are I think 7 places in tslibs where we do this iteration.  Can they all be optimized? Another perf improvement waiting in these files is calling searchsorted via `cnp.PyArray_SearchSorted` instead of `arr.searchsorted`, since the latter is a python-space call.  Doing this is at least 3 kinds of a hassle so I never implemented it myself.  Feel free to ping me if you're interested in giving it a go. a bit -1 on actually using the numpy c-api for searchsorted as it adds complexity; I suspect it won't actually speed things up for us (and is out of scope for this PR anyhow). > a bit -1 on actually using the numpy c-api for searchsorted as it adds complexity; I suspect it won't actually speed things up for us (and is out of scope for this PR anyhow).

Definitely agree on the scope, and like I said it is a PITA.  But it's also one of the few python-space calls left in these files that we do at each stage of a loop. > Definitely agree on the scope, and like I said it is a PITA. But it's also one of the few python-space calls left in these files that we do at each stage of a loop.

yep. @jbrockmendel Not all, but at least one or two more. I just got back into town and have some errands to run, so will have new asv results this evening.

I have an (unsubmitted) PR for `np.searchsorted` to fix the needle/haystack-have-different-dtypes performance issue, so might be in a position to investigate switching to the c-api after I finish cleaning that up. @qwhelan can you update (small comment), ping on green. @qwhelan can you merge master and update that comment @mroeschke Looks like the `tzlocal` benchmark improves a fair bit:
```
       before           after         ratio
     [02a97c0a]       [d20f6fac]
     <tz_aware_op_speedup~4>       <tz_aware_op_speedup>
-      11.9±0.01s       5.77±0.01s     0.48  timeseries.DatetimeIndex.time_add_timedelta('tz_local')
```

But it's still ~200x slower:
```
[ 53.12%] ··· timeseries.DatetimeIndex.time_add_timedelta                                                                                                                 ok
[ 53.12%] ··· ============ =============
               index_type
              ------------ -------------
                  dst         471±8μs
                repeated    3.52±0.05ms
                tz_aware    3.78±0.07ms
              **tz_local     5.77±0.01s**
                tz_naive    1.98±0.03ms
              ============ =============
```

@jreback digging into a regression I thought was spurious, will have an update in an hour or two sure! @jreback Good to merge, can't reproduce the spurious regression. thanks @qwhelan keep em coming!