Profile and improve vbucket compaction performance

This ticket is to track the effort to profile and improve the performance of compaction. Since memcached now logs compactions that take longer than 1s, we have greater visibility into how long compactions are taking. Subsequently we've had different logs from different customers where a 500MB vbucket has taken more than 1 minute to compact. a 10GB vbucket has taken 300s to compact. In both instances the understanding is performance is not bound by the performance of the underlying storage.