Delete CRIU checkpoint on /bau delete

/bau delete only removed the world folder. The checkpoint of the build
server stayed in place, so the next start restored the old JVM state
against a freshly copied world (superflat world, plugins inactive).
Extract ServerStarter#checkpointDir and delete the checkpoint together
with the world.
Fixes #438

VelocityCore/src/de/steamwar/velocitycore/ServerStarter.java

@@ -303,7 +303,7 @@ public class ServerStarter {
                 worldSetup.run();
                 arguments.put("logPath", worldName);
 
-                File checkpointDir = new File("/tmp/" + System.getProperty("user.name") + ".checkpoints/" + directory.getName() + "/" + worldName);
+                File checkpointDir = checkpointDir(directory, worldName);
                 if (checkpoint) {
                     arguments.put("checkpoint", checkpointDir.getPath());
                 }
@@ -326,6 +326,10 @@ public class ServerStarter {
         }).schedule();
     }
 
+    public static File checkpointDir(File serverDirectory, String worldName) {
+        return new File("/tmp/" + System.getProperty("user.name") + ".checkpoints/" + serverDirectory.getName() + "/" + worldName);
+    }
+
     private void regularStart(String serverName, int port) {
         postStart(constructor.construct(serverName, port, node.startServer(
                 version, directory, worldDir, worldName, port, arguments.entrySet().stream().map(entry -> entry.getKey() + "=" + entry.getValue()).toArray(String[]::new)

VelocityCore/src/de/steamwar/velocitycore/commands/BauCommand.java

@@ -261,6 +261,7 @@ public class BauCommand extends SWCommand {
                 }
 
                 SubserverSystem.deleteFolder(VelocityCore.local, world);
+                SubserverSystem.deleteFolder(VelocityCore.local, ServerStarter.checkpointDir(version.getServerDirectory("Bau"), String.valueOf(sender.user().getId())).getPath());
                 sender.system("BAU_DELETE_DELETED");
             }).schedule();