Summary:
Moved the logic for core-local array out of ConcurrentArena and into a separate class because I want to reuse it for core-local stats.
Closes https://github.com/facebook/rocksdb/pull/2256
Differential Revision: D5011518
Pulled By: ajkr
fbshipit-source-id: a75a7b8f7b7a42fd6273489ada405f14c6be196a