Called when asyncio.Protocol loses the network connection.

def connection_lost(self, exc):
        """Called when asyncio.Protocol loses the network connection."""
        if exc is None:
            self.log.warning('eof from receiver?')
        else:
            self.log.warning('Lost connection to receiver: %s', exc)

        self.transport = None

        if self._connection_lost_callback:
            self._loop.call_soon(self._connection_lost_callback)