Integration with and wrapper around ae.sys.shutdown for networked (ae.net.asockets-based) applications.
Unlike ae.sys.shutdown, the handlers are called from within the same thread they were registered from - provided that socketManager.loop() is running in that thread.
Register a handler to be called when a shutdown is requested. The handler should close network connections and cancel timers, thus removing all owned resources from the event loop which would block it from exiting cleanly.
Calls all registered handlers.