Summary:
We estimate number of reads per SST files, by updating the counter per file in sampled read requests. This information can later be used to trigger compactions to improve read performacne.
Closes https://github.com/facebook/rocksdb/pull/2417

Differential Revision: D5193528

Pulled By: siying

fbshipit-source-id: b4241c5ad0eaf444b61afb53f8e6290d9f5da2df