implement _local_timestamps in DatetimeArrayMixin
Broken out of #21715 because it is a blocker for a handful of orthogonal follow-ons.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=h1) Report
> Merging [#21721](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9b247706a907acea3af271d82fd75e98bcbc924f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21721/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21721      +/-   ##
==========================================
+ Coverage   91.93%   91.93%   +<.01%     
==========================================
  Files         159      159              
  Lines       49719    49722       +3     
==========================================
+ Hits        45707    45710       +3     
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (ø)` | :arrow_up: |
| #single | `42.02% <27.27%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21721/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21721/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.45% <100%> (-0.03%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=footer). Last update [9b24770...70e4047](https://codecov.io/gh/pandas-dev/pandas/pull/21721?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
