MB-10033 vbucket compaction via ep-engine

ns_server issues a memcached api call of CMD_COMPACT_DB
this is a blocking call and compaction of the next
vbucket will start only after this call returns

ns_server also listens to the new mccouch notification
indicating completion of compaction by ep-engine.

after the notification is received ns_server adjusts
the file version in couchdb

master vbucket is still compacted the old way by
couchdb compactor

Change-Id: I978a65ca8df5418dda6c6d552f47c97c4fdd1e41
Reviewed-on: http://review.couchbase.org/30226
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>