MB-11259 Change bytes field in upr buffer ack message

The protocol message for buffer ack is modified to have
non-zero extlen.

Change-Id: Ia5a355b55f2e27ece494983e62ac3cd04a026889
Reviewed-on: http://review.couchbase.org/38736
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>