Disconnect after timeout

[randomPoison]

If a connection goes too long without receiving any data from the other side, then we should consider the connection as having disconnected. This means removing it from the open_connections map, and having the Client's Stream implementation start returning None when polled.

[randomPoison]

This was partially implemented in #26, but this will be much easier to fully implement after #24. Having the message pump centralized would also allow us to centralize the timeout, whereas right now the issue is that there are multiple places where the timeout should be checked but isn't.