create the server endpoint.

async def start_serving(self, address=None, sockets=None, **kw):
        """create the server endpoint.
        """
        if self._server:
            raise RuntimeError('Already serving')
        server = DGServer(self._loop)
        loop = self._loop
        if sockets:
            for sock in sockets:
                transport, _ = await loop.create_datagram_endpoint(
                    self.create_protocol, sock=sock)
                server.transports.append(transport)
        elif isinstance(address, tuple):
            transport, _ = await loop.create_datagram_endpoint(
                self.create_protocol, local_addr=address)
            server.transports.append(transport)
        else:
            raise RuntimeError('sockets or address must be supplied')
        self._set_server(server)