Summary:
NotifyOnCompactionCompleted can unlock the mutex.
That mean that we can schedule a background compaction that will start before we ReleaseCompactionFiles().

Test Plan:
added unittest
existing unittest

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: yoshinorim, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58065