Summary:
Add `compaction_reason` as part of event log for event `compaction started`.
Add counters for each `CompactionReason`.
Closes https://github.com/facebook/rocksdb/pull/3679

Differential Revision: D7550348

Pulled By: riversand963

fbshipit-source-id: a19cff3a678c785aa5ef41aac78b9a5968fcc34d