MB-10488: Add a notifier upr connection type

The upr notifier connection is used to get updates through an upr
stream about when new items are available to stream.

Change-Id: I276e558bf36c43f427d221351495587ee5063154
Reviewed-on: http://review.couchbase.org/34572
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>