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
ac39d6bec5