Connections leak when closed by the client

[Tragetaschen]

I'm currently tracking down ways the loop resource handle's Finalizer from #60 is called instead of the Dispose method.

One occurrence is when a Connection: Keep-Alive session is closed by the client instead of being reused.

[Tragetaschen]

Thinking about it: The code is not really doing anything special when waiting for the next Frame on a connection as opposed to any other read operation during request handling.

This explains the large number of connections when killing ab in #34.