Unsubscribe on agent disconnection

[bensheldon]

This commit fixes #14 by tracking subscriptions in the Agent model and unsubscribing from those channels on disconnection.

BUT, it now presents another issue: once a channel's subscribers drops to zero, subsequent new subscribers to that channel are no longer able to send messages. I'm not sure if that's an issue with InMemoryMQ or somewhere else, but I thought I'd open the pull request to see if you had any ideas.

[aaronblohowiak]

Tracking the subscriptions in Agent makes some sense, but we'd also need to track unsubs there to keep it coherent. I will think more about this later tonight, but my current thoughts are that this should be stored in the SubscriptionManager.. either way, your unsub agent on disconnect does need to notify the subscriptionmanager that the agent should be considered dead.

For the channel's subscribers dropping to zero, I need to add a delete this.subscriptions[chan] inside the SubscriptionManager.prototype.unsubscribe function on the condition that the subscription count drops to zero -- it is unsubscribing from the mq (which is correct) but it did not perform the final clean-up. Good catch.

These issues are coming up because of the recent re-factor to use SockJS -- socket.io would allow us to send messages to disconnected clients without complaint (which I consider an anti-feature.)

Sorry for the instability, I will add more tests to cover these cases soon.

[bensheldon]

This PR seems pretty stale. Closing it out.