WebSocket in cluster mode

### Description

I see ClientSocket.broadcast is just call channel.rebroadcast!

```
protected def rebroadcast!(message)
    subscribers = ClientSockets.get_subscribers_for_topic(message["topic"])
    subscribers.each_value(&.socket.send(message.to_json))
end
```

when it runs in cluster mode, the socket in other process which subscribe the same topic can not receive the message
