Write thread.

def write_daemon(self):
        """Write thread."""
        while True:
            (message, callback) = self._write_queue.get(block=True)
            self.logger.info("Sending message on USB bus: %s", str(message))
            self.logger.debug("Sending binary message:  %s", str(message.to_binary()))
            self._reader.write(message.to_binary())
            time.sleep(self.SLEEP_TIME)
            if callback:
                callback()