MB-10882 open upr connection for upr_notifier only when the first

.. subscribe request arrives. That prevents upr_notifier from repeatedly
crashing while memcached is not available yet after the server start
and after memcached is shut down on server shutdown

Change-Id: Ifb7a661d50ec2d85ec27b575f1fe9cb41d884659
Reviewed-on: http://review.couchbase.org/36023
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>