Try to flush buffered data to the SCTP layer.

        We wait until the association is established, as we need to know
        whether we are a client or a server to correctly assign an odd/even ID
        to the data channels.

async def _data_channel_flush(self):
        """
        Try to flush buffered data to the SCTP layer.

        We wait until the association is established, as we need to know
        whether we are a client or a server to correctly assign an odd/even ID
        to the data channels.
        """
        if self._association_state != self.State.ESTABLISHED:
            return

        while self._data_channel_queue and not self._outbound_queue:
            channel, protocol, user_data = self._data_channel_queue.popleft()

            # register channel if necessary
            stream_id = channel.id
            if stream_id is None:
                while self._data_channel_id in self._data_channels:
                    self._data_channel_id += 2
                stream_id = self._data_channel_id
                self._data_channels[stream_id] = channel
                self._data_channel_id += 2
                channel._setId(stream_id)

            # send data
            if protocol == WEBRTC_DCEP:
                await self._send(stream_id, protocol, user_data)
            else:
                if channel.maxPacketLifeTime:
                    expiry = time.time() + (channel.maxPacketLifeTime / 1000)
                else:
                    expiry = None
                await self._send(stream_id, protocol, user_data,
                                 expiry=expiry,
                                 max_retransmits=channel.maxRetransmits,
                                 ordered=channel.ordered)
                channel._addBufferedAmount(-len(user_data))