Summary:
ReadOptions.fill_cache is set in compaction inputs and can be set by users in their queries too. It tells RocksDB not to put a data block used to block cache.

The memory used by the data block is, however, not trackable by users.

To make the system more manageable, we can cost the block to block cache while using it, and then release it after using.
Closes https://github.com/facebook/rocksdb/pull/3333

Differential Revision: D6670230

Pulled By: miasantreble

fbshipit-source-id: ab848d3ed286bd081a13ee1903de357b56cbc308