Compaction for MM runs Cleaner - needs test once IOW is supported

Deletion of aborted deltas happens from CompactorMR.run() i.e. from Worker
but the Worker still sets compaction_queue entry to READY_FOR_CLEANING.
This is not needed if there are no base_N dirs which can be created by Insert Overwrite

In this case we can't delete deltas < N until we know no one is reading them, i.e. in Cleaner