CBD-485: Make CMD_COMPACT_DB call synchronous

NS-Server needs the CMD_COMPACT_DB command to block in ep-engine
until the compaction actually completes.
So this change blocks the connection in EWOULDBLOCK state and
the connection, on compaction completion, gets notified to return SUCCESS
This is in anticipation of the removal of the MCCouch notification
Change-Id: I462d4a591bf1480ac2733468ec0981ed3a4c81a7
Reviewed-on: http://review.couchbase.org/33008
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>