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?).

[max-tern]

my setup:

The root supervisor monitors some workers and one wisp/mist web server.
Our workers added to a root static_supervisor as worker_child since wisp web server re-export (return?) glisten supervisor it is added as supervisor_child

supervisor_child spec has infinite timeout to shutdown.

When my work is down and cannot be recovered, the shutdown message sent to glisten and it is ignored as the supervisor mod is not OTP compliant (as I understand did not handle all system msg) causing all supervisor tree to hang in "waiting for supervisor shutdown" state if any of my workers are down.

Also, this prevents my OTP application from graceful shutdown.

More details are here

A big step forward will be to migrate to static_supervisor.