Limit queue sizes

[talex5]

Currently there is no bound on the size of the queue in Endpoint. A client that keeps sending requests and never reads responses will keep using up more and more memory in the server.

We should add some stress tests and try to handle this. Dropping individual messages could break E-order, so we should probably drop the entire connection. We could also try not reading more input until the output has been consumed, although on its own this could deadlock.