Fix disconnect logic for upr connections

When an upr connection is disconnected we should immediately
remove that connection from the upr connection map.

Change-Id: Ic4c7615adadb30952d261b6e6dd56576c7306cff
Reviewed-on: http://review.couchbase.org/30025
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>