MB-8413 Release CouchNotifier instances if not referenced

Change-Id: I88766f6e6da14107c0bb712b4d732448f5cc742d
Reviewed-on: http://review.couchbase.org/26845
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>