OF-2437: Introduce CachingPubsubPersistenceProviderTest

This new unit test verifies behavior of CachingPubsubPersistenceProvider (which fails, as indicated by some of these test failing).

The tests introduced here only cover one (of three) types of operations that are being scheduled by the implementation:
- Node creation/changing/removal

These two other types of operation still need to be added:
- Affiliation creation/update/removal
- Subscription creation/update/removal

xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/CachingPubsubPersistenceProvider.java

@@ -15,6 +15,7 @@
  */
 package org.jivesoftware.openfire.pubsub;
 
+import com.google.common.annotations.VisibleForTesting;
 import org.jivesoftware.openfire.cluster.ClusterManager;
 import org.jivesoftware.openfire.pep.PEPService;
 import org.jivesoftware.openfire.pubsub.cluster.FlushTask;
@@ -82,7 +83,8 @@ public class CachingPubsubPersistenceProvider implements PubSubPersistenceProvid
      */
     private final HashMap<PublishedItem.UniqueIdentifier, PublishedItem> itemsPending = new HashMap<>();
 
-    private ConcurrentMap<Node.UniqueIdentifier, List<NodeOperation>> nodesToProcess = new ConcurrentHashMap<>();
+    @VisibleForTesting
+    final ConcurrentMap<Node.UniqueIdentifier, List<NodeOperation>> nodesToProcess = new ConcurrentHashMap<>();
 
     /**
      * Cache name for recently accessed published items.