PERF/CLN: infer Period in infer_dtype, later index inference is faster

[jreback]

This was meant to fix the vbench: ctor_index_array_string

see #3326

not sure if this is that stable of a perf fix

-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
frame_get_dtype_counts                       |   0.0960 |   0.1700 |   0.5647 |
frame_insert_500_columns                     |  87.8456 | 142.5257 |   0.6163 |
frame_iteritems_cached                       |   0.0820 |   0.1043 |   0.7860 |
stat_ops_level_frame_sum                     |   2.5051 |   2.8849 |   0.8683 |
ctor_index_array_string                      |   0.0333 |   0.0383 |   0.8693 |
stat_ops_level_series_sum_multiple           |   5.6090 |   6.4037 |   0.8759 |
frame_fancy_lookup                           |   2.7517 |   3.1263 |   0.8802 |

Target [f3b19d6] : PERF/CLN: infer Period in infer_dtype, later index inference is faster
Base   [df2c21c] : Merge pull request #3327 from jreback/index_perf