Services can register themselves with the GracefulShutdownService to
make sure they will be stopped gracefully on server shutdown.

This allows plugins to participate in a graceful server shutdown to
make sure the server is not terminated before the plugin shutdown
routine is done.

Once the shutdown has started, we cannot modify the set of hooks because
that might to race conditions and a hanging shutdown.

Objects that try to register/unregister during the shutdown phase will
get an IllegalStateException that should be handled.

This will make it easier for users to debug a hanging shutdown process.

We don't really need a blocking latch in the hook because we can also
check the behavior after the service is terminated.

The latch is open at this point and all shutdown hooks have been
executed.