Transmit a chunk (no bundling for now).

async def _send_chunk(self, chunk):
        """
        Transmit a chunk (no bundling for now).
        """
        self.__log_debug('> %s', chunk)
        await self.__transport._send_data(serialize_packet(
            self._local_port,
            self._remote_port,
            self._remote_verification_tag,
            chunk))