Summary:
This patch enables using PinnableSlice for RowCache, changes include
not releasing the cache handle immediately after lookup in TableCache::Get, instead pass a Cleanble function which does Cache::RleaseHandle.
Closes https://github.com/facebook/rocksdb/pull/2492

Differential Revision: D5316216

Pulled By: maysamyabandeh

fbshipit-source-id: d2a684bd7e4ba73772f762e58a82b5f4fbd5d362