Summary:
Split out interfaces needed for blob storage from #1560, including
* CompactionEventListener and OnFlushBegin listener interfaces.
* Blob filename support.
Closes https://github.com/facebook/rocksdb/pull/2169

Differential Revision: D4905463

Pulled By: yiwu-arbug

fbshipit-source-id: 564e73448f1b7a367e5e46216a521e57ea9011b5