Consider getting rid of client tracking about engine.io

[rauchg]

By abstract I mean that accessing a client by session id would imply not accessing a object that's tied to a transport, but accessing to a "communication layer" with the client.

[nicokaiser]

I'll add the thoughts we discussed about ømq to this issue:

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Idea:

Sessions could be sticked to processes (e.g. the process that gets the initial handshake from the client is responsible for the session and handles it – "only" client communication is done by pubsub. With 1. however this could lead to very inefficient distribution (many clients are connected to processes that don't actually handle their requests but only forward them to a different one).

• instead of Socket.writeBuffer, write message to queue (ømq or RabbitMQ)
• transport is closed (timeout, close request) => delete queue
• transport becomes writable (or another process gets a poll for an 'unknown' socket id) => queue is flushed to this client (no such queue => session id is invalid)

Are "persistent channels" possible in ømq? I.e. channels you can write to with the subscribers eventually appering later? Is this even possible without a message broker like RabbitMQ?

[rauchg]

We're going to make it optional instead.