Check in queue if still in stock

Thanks for working on this.

Why are we setting sent_time in case the notification is not being sent?

+      if (!$checker->getIsInStock($product, $locations)) {
+        $time = new DrupalDateTime();
+        $data->set('sent_time', $time->getTimestamp())->save();
+        return;
+      }

I think the correct behavior would be to just return, to remove the item from queue, as it is not valid anymore.

I converted the patch from #3 to MR and implemented the suggestion from #4. Committed and pushed this. Thanks!