MB-9898: Add flow control to the upr producer

Change-Id: Ia94099d249c0624d153be948ac085163608de9db
Reviewed-on: http://review.couchbase.org/35431
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>