Summary: RocksDB 4.11 has been branched out. Now need to move to 4.12.

Test Plan: N/A

Reviewers: kradhakrishnan, andrewkr, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61845