reset tick rate when changing worlds in SP

src/main/java/carpet/CarpetServer.java

@@ -118,6 +118,7 @@ public class CarpetServer // static for now - easier to handle all around the co
         if (scriptServer != null) scriptServer.onClose();
         settingsManager.detachServer();
         LoggerRegistry.stopLoggers();
+        TickSpeed.reset();
         extensions.forEach(e -> e.onServerClosed(server));
         minecraft_server = null;
     }