Summary:
SST Partitioner interface that allows to split SST files during compactions.

It basically instruct compaction to create a new file when needed. When one is using well defined prefixes and prefixed way of defining tables it is good to define also partitioning so that promotion of some SST file does not cover huge key space on next level (worst case complete space).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/6957

Reviewed By: ajkr

Differential Revision: D22461239

fbshipit-source-id: 9ce07bba08b3ba89c2d45630520368f704d1316e