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: I8f568f7696b09f64703b9bab50a9c5420c9d0c80
Reviewed-on: http://review.couchbase.org/34868
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: buildbot <build@couchbase.com>