MB-11640: Allow clients to set their own dcp priority

Change-Id: Ibeea29dbcef678fb76368e1dc6cc58ff4d7fb88d
Reviewed-on: http://review.couchbase.org/43135
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>