PERF: fix getitem unique_check / initialization issue
closes #14930 
```
    before     after       ratio
  [708792aa] [a46287a8]
-  169.39ms   153.27ms      0.90  panel_ctor.Constructors1.time_panel_from_dict_all_different_indexes
-   17.17ms    15.36ms      0.89  join_merge.Join.time_join_dataframe_index_multi
-    9.92ms     8.87ms      0.89  index_object.StringIndex.time_boolean_indexer
-    9.17ms     8.18ms      0.89  reindex.LibFastZip.time_lib_fast_zip
-  822.04μs   731.27μs      0.89  reindex.LevelAlign.time_reindex_level
-    3.10ms     2.75ms      0.89  indexing.IndexingMethods.time_take_dtindex
-   63.11ms    55.97ms      0.89  indexing.Int64Indexing.time_ix_list_like
-  316.38μs   280.35μs      0.89  reindex.FillMethod.time_pad
-    4.31ms     3.82ms      0.88  index_object.SetOperations.time_str_symmetric_difference
-   10.10ms     8.88ms      0.88  index_object.Multi3.time_datetime_level_values_full
-    9.96μs     8.65μs      0.87  indexing.IndexingMethods.time_get_loc_float
-   40.57ms    35.20ms      0.87  reindex.Duplicates.time_frame_drop_dups_int
-   65.57ms    56.86ms      0.87  indexing.Int64Indexing.time_loc_scalar
-   62.76ms    54.24ms      0.86  indexing.Int64Indexing.time_ix_array
-   63.77ms    54.83ms      0.86  indexing.Int64Indexing.time_getitem_array
-  433.07ms   370.61ms      0.86  index_object.Multi2.time_sortlevel_int64
-   10.41ms     8.84ms      0.85  index_object.StringIndex.time_boolean_series_indexer
-   63.09ms    53.41ms      0.85  indexing.Int64Indexing.time_getitem_list_like
-    1.55ms     1.31ms      0.84  reindex.Reindexing.time_reindex_multiindex
-   65.60ms    54.52ms      0.83  indexing.Int64Indexing.time_loc_list_like
-  110.24ms    91.35ms      0.83  index_object.Multi1.time_duplicated
-  132.45ms   107.34ms      0.81  join_merge.Align.time_series_align_int64_index
-     2.62s      2.10s      0.80  join_merge.JoinIndex.time_left_outer_join_index
-   63.71ms    50.74ms      0.80  reindex.Align.time_align_series_irregular_string
-    3.35ms     2.65ms      0.79  reindex.Duplicates.time_frame_drop_dups_na_inplace
-  249.99ms   191.72ms      0.77  index_object.SetOperations.time_int64_symmetric_difference
-    2.49ms     1.86ms      0.75  index_object.SetOperations.time_str_difference
-  307.17ms   224.61ms      0.73  indexing.StringIndexing.time_getitem_label_slice
-  304.70ms   215.89ms      0.71  indexing.StringIndexing.time_get_value
-  178.70ms   126.61ms      0.71  index_object.SetOperations.time_int64_difference
-  566.26μs   379.90μs      0.67  reindex.Reindexing.time_reindex_dates
-  141.99ms    94.44ms      0.67  index_object.SetOperations.time_int64_union
-   22.77μs    15.10μs      0.66  indexing.DataFrameIndexing.time_getitem_scalar
-    6.49ms     4.28ms      0.66  index_object.SetOperations.time_datetime_symmetric_difference
-    6.14ms     3.83ms      0.62  index_object.SetOperations.time_datetime_difference
-   95.17ms    46.62ms      0.49  index_object.SetOperations.time_int64_intersection
-    4.09ms     1.60ms      0.39  index_object.SetOperations.time_datetime_difference_disjoint
-  275.79μs   103.85μs      0.38  index_object.SetOperations.time_datetime_union
-     3.30s   223.13ms      0.07  frame_methods.Iteration.time_iteritems_indexing

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
``` @jreback am I correct in thinking that the main functional change here is the addition of the second clause in the boolean check [here?](https://github.com/pandas-dev/pandas/pull/14933/files#diff-4f3357a9f53e943087e2778134494905R264).  **EDIT:** that link doesn't work.  I'll comment inline. @ssanderson no I think was missing a ``unique_check``(since renamed to ``need_unique_check`` and I reversed the polarity of these.

The original reason for this (the lazy construction), should be preserved. (I wish we actually had a test for this :<)

The ``.mapping is None``  clause was to correct for a case where we hit ``_ensure_mapping_populated`` but we actually hadn't populated it yet. (note this *doesn't* actually happen when over threshold).

The ``needs_*`` checks are really just to make things work when you have uniques. > no I think was missing a unique_check

Yeah, I realized we'd actually flipped the polarity on a second read. Some day I will find the time to finish my branch that was working on this code :/.  Maybe I'll have a free night or two over the holidays. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14933?src=pr) is 84.64% (diff: 100%)
> Merging [#14933](https://codecov.io/gh/pandas-dev/pandas/pull/14933?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #14933   diff @@
==========================================
  Files           144        144          
  Lines         51025      51025          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43191      43191          
  Misses         7834       7834          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [4c3d4d4...dc32b39](https://codecov.io/gh/pandas-dev/pandas/compare/4c3d4d4fbb5e1093a324d81dfd9de5d7650012c6...dc32b391182a33be64d8295179afc645310ee7c3?src=pr) @ssanderson I simplified :+1: from me if tests are passing.