Revert "Use spinlocks instead of conn mutex to guard UPR notifications"

This reverts commit 6003f370772d611fa7981e84bb135911761176f8.

Change-Id: I1affd06e7c02b43e3ad18f498b2f891fea9466c4
Reviewed-on: http://review.couchbase.org/36696
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>