Add stop-server procedure to spiffy

[sjamaan]

In some cases, it can be useful to gracefully stop accepting new connections so that the main thread can return. Useful when spawning a temporary server or so.

[sjamaan]

With the current design, this is very difficult; we can't stop a "hanging" accept call, which is what Spiffy does immediately upon entering the accept loop. When a request is handled, it fires off a thread to handle the request and loops to accept the next incoming request.