Renamed AbstractListenerRegistry to ListenerRegistry (since it is not abstract).
- added checkForNonRegistrant() predicate and augmented unit tests.

Change-Id: I7ecc618f01fdc4cead6f143a1e5e0ddd03268efc
diff --git a/core/net/src/main/java/org/onosproject/cluster/impl/ClusterManager.java b/core/net/src/main/java/org/onosproject/cluster/impl/ClusterManager.java
index b08c5ca..c8ffe88 100644
--- a/core/net/src/main/java/org/onosproject/cluster/impl/ClusterManager.java
+++ b/core/net/src/main/java/org/onosproject/cluster/impl/ClusterManager.java
@@ -32,7 +32,7 @@
 import org.onosproject.cluster.ClusterStoreDelegate;
 import org.onosproject.cluster.ControllerNode;
 import org.onosproject.cluster.NodeId;
-import org.onosproject.event.AbstractListenerRegistry;
+import org.onosproject.event.ListenerRegistry;
 import org.onosproject.event.EventDeliveryService;
 import org.slf4j.Logger;
 
@@ -54,8 +54,8 @@
 
     private ClusterStoreDelegate delegate = new InternalStoreDelegate();
 
-    protected final AbstractListenerRegistry<ClusterEvent, ClusterEventListener>
-            listenerRegistry = new AbstractListenerRegistry<>();
+    protected final ListenerRegistry<ClusterEvent, ClusterEventListener>
+            listenerRegistry = new ListenerRegistry<>();
 
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected ClusterStore store;