Don't keep managing two rocksdb version

Summary:
Before this diff, there are two places with rocksdb versions. After the diff:
1. we only have one source of truth for rocksdb version
2. we have a script that we can use to get the version that we can use in other compilations (java, go, etc).

Test Plan: make

Reviewers: yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24333