Summary:
The patch cleans up the handling of cache hit/miss/insertion related
performance counters, get context counters, and statistics by
eliminating some code duplication and factoring out the affected logic
into separate methods. In addition, it makes the semantics of cache hit
metrics more consistent by changing the code so that accessing a
partition of partitioned indexes/filters through a pinned reference no
longer counts as a cache hit.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5408

Differential Revision: D15610883

Pulled By: ltamasi

fbshipit-source-id: ee749c18965077aca971d8f8bee8b24ed8fa76f1