Ensure DataLoaderSubscriber is only called by one thread

Multiple threads may call `onNext` - we thus (lazily) chuck a
`synchronized` to ensure correctness at the cost of speed.

In future, we should examine how we should manage this concurrency
better.

Author: AlexandreCarlton

src/main/java/org/dataloader/DataLoaderHelper.java

@@ -648,8 +648,10 @@ public void onSubscribe(Subscription subscription) {
             subscription.request(keys.size());
         }
 
+        // onNext may be called by multiple threads - for the time being, we pass 'synchronized' to guarantee
+        // correctness (at the cost of speed).
         @Override
-        public void onNext(V value) {
+        public synchronized void onNext(V value) {
             assertState(!onErrorCalled, () -> "onError has already been called; onNext may not be invoked.");
             assertState(!onCompleteCalled, () -> "onComplete has already been called; onNext may not be invoked.");