graceful shutdown: ./shutdown flushes players, then stops the servers

The stock scripts stop the game with SIGKILL (mbkill.sh runs kill -9) and
'./reboot' only calls mbrestart.sh, so anything still parked in a character's
deferred database job is thrown away: experience is written five minutes after
it is earned and stats/skills thirty seconds after they change
(engine.jobs.DatabaseUpdateJob). PlayerCharacter.updateDatabase() is an empty
stub, so there was no write-out path at all.

Added engine.gameManager.GracefulShutdown:
  flushAllPlayers()      - writes skills/powers, stat modifiers and experience
                           for every online character
  announce(String)       - server wide flash message
  registerShutdownHook() - flush on an orderly SIGTERM (SIGKILL cannot be
                           intercepted by design)
  gracefulStop(int)      - countdown, flush, then stop the login and world
                           servers

Added the './shutdown [seconds]' dev command (engine.devcmd.cmds.ShutdownCmd,
default 60s, ADMIN only like every other dev command), registered together with
the shutdown hook in WorldServer.main and LoginServer.main.

Verified on a MagicBox container: ant builds clean, the boot log shows
"shutdown hook registered for WorldServer"/"LoginServer", and kill -TERM on the
world server logs "WorldServer is terminating, flushing player data" ->
"flush finished, saved=0 failed=0" -> clean exit.

src/engine/devcmd/cmds/ShutdownCmd.java

@@ -0,0 +1,84 @@
+// • ▌ ▄ ·.  ▄▄▄·  ▄▄ • ▪   ▄▄· ▄▄▄▄·  ▄▄▄·  ▐▄▄▄  ▄▄▄ .
+// ·██ ▐███▪▐█ ▀█ ▐█ ▀ ▪██ ▐█ ▌▪▐█ ▀█▪▐█ ▀█ •█▌ ▐█▐▌·
+// ▐█ ▌▐▌▐█·▄█▀▀█ ▄█ ▀█▄▐█·██ ▄▄▐█▀▀█▄▄█▀▀█ ▐█▐ ▐▌▐▀▀▀
+// ██ ██▌▐█▌▐█ ▪▐▌▐█▄▪▐█▐█▌▐███▌██▄▪▐█▐█ ▪▐▌██▐ █▌▐█▄▄▌
+// ▀▀  █▪▀▀▀ ▀  ▀ ·▀▀▀▀ ▀▀▀·▀▀▀ ·▀▀▀▀  ▀  ▀ ▀▀  █▪ ▀▀▀
+//      Magicbane Emulator Project © 2013 - 2022
+//                www.magicbane.com
+
+
+package engine.devcmd.cmds;
+
+import engine.devcmd.AbstractDevCmd;
+import engine.gameManager.GracefulShutdown;
+import engine.objects.AbstractGameObject;
+import engine.objects.PlayerCharacter;
+
+/**
+ * Devcmd: graceful shutdown.
+ *
+ * Announces a countdown to every player, writes all online characters to the
+ * database (skills/powers, stat modifiers, experience) and then stops the
+ * login and world servers. Unlike './reboot' - which simply runs mbrestart.sh
+ * and therefore kills the JVMs with SIGKILL - nothing still parked in a
+ * deferred database job is lost.
+ *
+ * Usage: ./shutdown [seconds]  (default 60, maximum 3600)
+ */
+public class ShutdownCmd extends AbstractDevCmd {
+
+    private static final int DEFAULT_DELAY_SECONDS = 60;
+    private static final int MAX_DELAY_SECONDS = 3600;
+
+    public ShutdownCmd() {
+        super("shutdown");
+    }
+
+    @Override
+    protected void _doCmd(PlayerCharacter pcSender, String[] args, AbstractGameObject target) {
+
+        int delay = DEFAULT_DELAY_SECONDS;
+
+        if (args.length > 0 && args[0].isEmpty() == false) {
+            try {
+                delay = Integer.parseInt(args[0].trim());
+            } catch (NumberFormatException e) {
+                this.sendUsage(pcSender);
+                return;
+            }
+        }
+
+        if (delay < 0)
+            delay = 0;
+
+        if (delay > MAX_DELAY_SECONDS)
+            delay = MAX_DELAY_SECONDS;
+
+        throwbackInfo(pcSender, "Graceful shutdown started: player data will be saved in " + delay + " second(s).");
+
+        final int delaySeconds = delay;
+
+        // The countdown sleeps, so run it off the message handling thread.
+
+        Thread shutdownThread = new Thread(new Runnable() {
+
+            @Override
+            public void run() {
+                GracefulShutdown.gracefulStop(delaySeconds);
+            }
+        }, "gracefulShutdown");
+
+        shutdownThread.setDaemon(false);
+        shutdownThread.start();
+    }
+
+    @Override
+    protected String _getHelpString() {
+        return "Gracefully shuts the server down: warns the players, saves every online character, then stops the servers";
+    }
+
+    @Override
+    protected String _getUsageString() {
+        return "' /shutdown [seconds]'";
+    }
+}