do not start upr_notifier for memcached buckets

This unbreaks memcached buckets support

Change-Id: I278fd48a274398e60a91ba904f6244ae43293b92
Reviewed-on: http://review.couchbase.org/35917
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>