The callback was triggered before the socket has actually joined/left
the room.

This follows #46. Each node will now listen to only three channels:

- `socket.io#<namespace>#*`: used when broadcasting
- `socket.io-request#<namespace>#`: used for requesting information (ex: get every room in the cluster)
- `socket.io-response#<namespace>#`: used for responding to requests

We keep the benefits of #46 since:

- messages from other namespaces are ignored
- when emitting to a single room, the message is sent to
  `socket.io#<namespace>#<my-room>`, so listeners can check whether they
  have the room before unpacking the message (which is CPU consuming).

But there is no need to subscribe / unsubscribe every time a socket
joins or leaves a room (which is also CPU consuming when there are
thousands of subscriptions).

Benchmark: https://github.com/darrachequesne/notepack#performance