MB-11698: Release reference to all connections in upr upon shutdown

Change-Id: I5759f89dcf1faaa5f7902fdb8fa08604a34f5814
Reviewed-on: http://review.couchbase.org/39514
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>