Summary:
This fixes the existing logic for pinning l0 index partitions. The patch preloads the partitions into block cache and pin them if they belong to level 0 and pin_l0 is set.

The drawback is that it does many small IOs when preloading all the partitions into the cache is direct io is enabled. Working for a solution for that.
Closes https://github.com/facebook/rocksdb/pull/2661

Differential Revision: D5554010

Pulled By: maysamyabandeh

fbshipit-source-id: 1e6f32a3524d71355c77d4138516dcfb601ca7b2