Use static_cast instead of dynamic_cast for upr responses

We have an event type so we can just do a static_cast to avoid
runtime type checking.

Change-Id: I0e87087241bc32ab910dc8a7b5a382b46fd0e31f
Reviewed-on: http://review.couchbase.org/31636
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>