Free vb_mutexes in ep.cc

Change-Id: I494c81b9dde49d69c1062cc62108b203d89e793e
Reviewed-on: http://review.couchbase.org/39744
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>