Summary:
It resets all the ticker and histogram stats to zero. Needed to change the locking a bit since Reset() is the only operation that manipulates multiple tickers/histograms together, and that operation should be seen as atomic by other operations that access tickers/histograms.
Closes https://github.com/facebook/rocksdb/pull/2213

Differential Revision: D4952232

Pulled By: ajkr

fbshipit-source-id: c0475c3e4c7b940120d53891b69c3091149a0679