Fix locking on the same lock that is used for loading causes the server to freeze

Author: TBlueF

BlueMapCommon/src/main/java/de/bluecolored/bluemap/common/BlueMapService.java

@@ -70,6 +70,7 @@
  */
 @DebugDump
 public class BlueMapService implements Closeable {
+
     private final ServerInterface serverInterface;
     private final BlueMapConfigProvider configs;
 
@@ -116,7 +117,7 @@ public String getWorldId(Path worldFolder) throws IOException {
         id = worldIds.get(worldFolder);
         if (id != null) return id;
 
-        synchronized (this) {
+        synchronized (worldIds) {
             // check again if another thread has already added the world
             id = worldIds.get(worldFolder);
             if (id != null) return id;