Refactor the way messages are retrieved from an upr producer

This cahnge adds in a ready queue that contains message waiting
to be sent over the wire for a given connection.

Change-Id: I1f60c2c37a98b6f9df9965e6803836ac4c0c7862
Reviewed-on: http://review.couchbase.org/30766
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>