Summary:
Port TimeseriesHistogram from common/stats to folly. Similarly to
MultiLevelTimeSeries and Histogram classes we've converted before.

Test Plan:
Ported the old unittest for TimeseriesHistogram from common/stats to
folly/test. The same unittest is left in tact and is still passing as well. Ran
all tests in folly/test and common/stats

Reviewed By: simpkins@fb.com

FB internal diff: D1054291