mdns.get_serial_number retries for no reason

The idea of "get_serial_number" in MiniDNS is that the Pool Manager can get the status of a zone on a nameserver at any time, synchronously, as a replacement for a cache value that it doesn't know.

https://github.com/openstack/designate/blob/master/designate/pool_manager/service.py#L648

This is going to be seriously hampered if, during the quick, "get me the serial", mdns compares the first serial it gets against
the one randomly hanging around in the zone object:
https://github.com/openstack/designate/blob/master/designate/mdns/notify.py#L130-L148

            if actual_serial is None or actual_serial < zone.serial:
                # TODO(vinod): Account for serial number wrap around.
                retries = retries - retry
                LOG.warn(_LW("Got lower serial for '%(zone)s' to '%(host)s:"
                             "%(port)s'. Expected:'%(es)d'. Got:'%(as)s'."
                             "Retries left='%(retries)d'") %
                         {'zone': zone.name, 'host': host,
                          'port': port, 'es': zone.serial,
                          'as': actual_serial, 'retries': retries})
                if retries > 0:
                    # retry again
                    time.sleep(retry_interval)
                    continue
                else:
                    break
            else:
                # Everything looks good at this point. Return SUCCESS.
                status = 'SUCCESS'
                break

This should simply return the first serial number it sees, because that's what the pool manager wants to know.

Unless I'm mistaken about the idea behind this function.