Improve test coverage for ConsistentMapEvent and WallClockTimestamp

Change-Id: Id8276966c227e2a66d40fca9ac43145b8a419982
diff --git a/core/api/src/main/java/org/onosproject/store/service/EventuallyConsistentMapEvent.java b/core/api/src/main/java/org/onosproject/store/service/EventuallyConsistentMapEvent.java
index f4ba859..fd27415 100644
--- a/core/api/src/main/java/org/onosproject/store/service/EventuallyConsistentMapEvent.java
+++ b/core/api/src/main/java/org/onosproject/store/service/EventuallyConsistentMapEvent.java
@@ -22,7 +22,7 @@
 /**
  * Representation of a EventuallyConsistentMap update notification.
  */
-public class EventuallyConsistentMapEvent<K, V> {
+public final class EventuallyConsistentMapEvent<K, V> {
 
     public enum Type {
         /**