MB-11721: Add a snapshot marker respnose api to upr_message_producers

We will be sending acks for some snapshot markers and need an api to
put the bytes on the wire.

Change-Id: Ib0a2d8443fd1fb800319fca9160f38df8661deae
Reviewed-on: http://review.couchbase.org/39378
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>