Reduce the lock contention in notifying UPR connections of an event

This change uses partitioned spinlocks to reduce the lock
contention in notifying UPR connections of mutation events.

Change-Id: I0e2898f8e10efcbf76ae200f7d9a0f42ebe4badd
Reviewed-on: http://review.couchbase.org/39254
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>