private void cleanChannel(final Channel channel) {
        //// todo ttl
        Singleton.INST.get(ChannelRepository.class).remove(channel);
    }
The DISCONNECT message is sent from the client to the server to indicate
that it is about to close its TCP/IP connection.

<p>This allows for a clean disconnection, rather than just dropping the line.
If the client had connected with the clean session flag set,
then all previously maintained information about the client will be discarded.

<p>A server should not rely on the client to close the TCP/IP connection after receiving a DISCONNECT.
shenyu-protocol/shenyu-protocol-mqtt/src/main/java/org/apache/shenyu/protocol/mqtt/Disconnect.java