Add support for graceful shutdown

[rawhat]

You should be able to send a message to the top-level supervisor. This should stop accepting new connections and allow existing connections to complete. If everything is complete, it should shut down. If anything goes longer than the specified time, we should forcefully bring the process down.

[rawhat]

Been randomly thinking about this off and on for, damn, I guess almost a year lol.

I think the approach would have to be something like this:

• Start a "manager" process which receives the acceptor processes when they (re)start
• Return some kind of opaque type Server instead of the raw Subject(supervisor.Message) to allow something like glisten.shutdown(server) and also glisten.get_supervisor(server) (otherwise there's no way to tell it to shutdown since the supervisor doesn't receive custom messages)
• When shutdown is received, send to all acceptors to shut down
• Also start some timer to force kill any processes that haven't shut down by then
• In the acceptor, if we receive the stop, close the socket and just do a normal stop

I think maybe we'll need a way to shut down the supervisor? Or otherwise tell it we're trying to shut down... need to test if normal shutdown will make the children restart (I would expect them to?).