MB-7966 per bucket singleton notifier

Change-Id: I4e249ca5ccc7fa632b7aaf488e1e89a3b9125fcd
Reviewed-on: http://review.couchbase.org/26202
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>