add back synchronized to setShouldStopAtCheckpoint to avoid problems

if start runs in parralel

Author: Hendrik Muhs

x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/TransformTask.java

@@ -292,7 +292,10 @@ synchronized void start(Long startingCheckpoint, ActionListener<StartTransformAc
      * @param shouldStopAtCheckpoint whether or not we should stop at the next checkpoint or not
      * @param shouldStopAtCheckpointListener the listener to return to when we have persisted the updated value to the state index.
      */
-    public void setShouldStopAtCheckpoint(boolean shouldStopAtCheckpoint, ActionListener<Void> shouldStopAtCheckpointListener) {
+    public synchronized void setShouldStopAtCheckpoint(
+        boolean shouldStopAtCheckpoint,
+        ActionListener<Void> shouldStopAtCheckpointListener
+    ) {
         logger.debug(
             "[{}] attempted to set task to stop at checkpoint [{}] with state [{}]",
             getTransformId(),