Get rid of mutex in CompactionJob's state

Summary: Based on @sdong's feedback in the diff, we shouldn't keep db_mutex in CompactionJob's state. This diff removes db_mutex from CompactionJob state, by making next_file_number_ atomic. That way we only need to pass the lock to InstallCompactionResults() because of LogAndApply()

Test Plan: make check

Reviewers: ljin, yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: sdong, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28491