[Magma] KV-Engine KVStore should be able to cancel a Magma compaction

KV-Engine KVStore should be able to cancel a Magma compaction both implicit and explicit. As there are situations where compaction can no longer be performed, for instance the in memory VBucket has been destroyed but Magma is trying to run an implicit compaction still. Currently we will just try and keep running the compaction on the vbucket which is a waste of time.