Force disconnect causes "In Use" flag to never be unset

[stephen]

e.g. Turning off wifi on iPad will cause nodetunes to refuse connections from other devices, thinking that the iPad is still connected.

[stephen]

In practice, I've seen OPTIONS being broadcast to the server ~1 request/second, but unsure if this is a standard heartbeat mechanism. If so, we could do something like:

socket.setTimeout(2000, this.disconnectHandler.bind({ self: this, socket: socket }));

But if this isn't the case, we'll be needlessly killing sessions.

[stephen]

Officially, we should use the data from the control port (https://github.com/stephen/nodetunes/blob/master/lib/rtp.js#L40) to determine a timeout.