Fix CRIU subserver stop

VelocityCore/Persistent/src/de/steamwar/persistent/Subserver.java

@@ -138,14 +138,16 @@ public class Subserver {
         writer.println(command);
     }
 
-    public void sleep() {
-        try {
-            long pid = process.pid();
+    private ProcessHandle serverProcess() {
         if (checkpoint) {
-                pid = process.children().findAny().map(ProcessHandle::pid).orElse(pid);
+            return process.children().findAny().orElseGet(process::toHandle);
+        }
+        return process.toHandle();
     }
 
-            Runtime.getRuntime().exec(new String[]{"kill", "-SIGUSR1", Long.toString(pid)});
+    public void sleep() {
+        try {
+            Runtime.getRuntime().exec(new String[]{"kill", "-SIGUSR1", Long.toString(serverProcess().pid())});
         } catch (IOException e) {
             logger.log(Level.SEVERE, "Failed to send SIGUSR1 to subserver.", e);
         }
@@ -162,7 +164,7 @@ public class Subserver {
 
     public void stop() {
         try {
-            process.destroy();
+            serverProcess().destroy();
             if (!process.waitFor(1, TimeUnit.MINUTES)) {
                 forceStop();
             } else if (thread.isAlive()) {
@@ -177,6 +179,7 @@ public class Subserver {
     public void forceStop() {
         try {
             logger.log(Level.SEVERE, () -> serverName + " did not stop correctly, forcibly stopping!");
+            serverProcess().destroyForcibly();
             process.destroyForcibly();
 
             if (thread.isAlive()) thread.join();