过期回调函数.

        如果设置了timeout则会启动一个协程按当前时间和最近的响应时间只差递归的执行这个回调

def timeout_callback(self):
        """过期回调函数.

        如果设置了timeout则会启动一个协程按当前时间和最近的响应时间只差递归的执行这个回调

        """
        # Check if elapsed time since last response exceeds our configured
        # maximum keep alive timeout value
        now = time()
        time_elapsed = now - self._last_response_time
        if time_elapsed < self.timeout:
            time_left = self.timeout - time_elapsed
            self._timeout_handler = (
                self._loop.call_later(
                    time_left,
                    self.timeout_callback
                )
            )
        else:
            logger.info('KeepAlive Timeout. Closing connection.')
            responseb = self.encoder({
                "MPRPC": self.VERSION,
                "CODE": 504
            })
            self._stream_writer.write(responseb)
            self.close()