[PATCH 3/3] Core-local statistics

[PATCH 3/3] Core-local statistics

Summary:
This diff changes `StatisticsImpl` from a thread-local approach to a core-local one. The goal is to perform faster aggregations, particularly for applications that have many threads. There should be no behavior change.
Closes https://github.com/facebook/rocksdb/pull/2258

Differential Revision: D5016258

Pulled By: ajkr

fbshipit-source-id: 7d4d165b4a91d8110f0409d113d1be91f22d31a9
(cherry picked from commit ac39d6bec5b2c23a2c3fd0f0e61d468be4f3e803)

Gbp-Pq: Name 0003-Core-local-statistics.patch