ONOS-1326: Added support for observing when node liveness status was last updated. Useful for detecting/debugging stability issues.

Change-Id: I8ffebcf3a09a51c6e3e7526986a0f05530ed757f
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 a7c3902..d9da6c6 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
@@ -15,12 +15,20 @@
  */
 package org.onosproject.cluster.impl;
 
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+import static org.slf4j.LoggerFactory.getLogger;
+
+import java.util.Set;
+
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Component;
 import org.apache.felix.scr.annotations.Deactivate;
 import org.apache.felix.scr.annotations.Reference;
 import org.apache.felix.scr.annotations.ReferenceCardinality;
 import org.apache.felix.scr.annotations.Service;
+import org.joda.time.DateTime;
+import org.onlab.packet.IpAddress;
 import org.onosproject.cluster.ClusterAdminService;
 import org.onosproject.cluster.ClusterEvent;
 import org.onosproject.cluster.ClusterEventListener;
@@ -31,15 +39,8 @@
 import org.onosproject.cluster.NodeId;
 import org.onosproject.event.AbstractListenerRegistry;
 import org.onosproject.event.EventDeliveryService;
-import org.onlab.packet.IpAddress;
 import org.slf4j.Logger;
 
-import java.util.Set;
-
-import static com.google.common.base.Preconditions.checkArgument;
-import static com.google.common.base.Preconditions.checkNotNull;
-import static org.slf4j.LoggerFactory.getLogger;
-
 /**
  * Implementation of the cluster service.
  */
@@ -97,6 +98,12 @@
         return store.getState(nodeId);
     }
 
+
+    @Override
+    public DateTime getLastUpdated(NodeId nodeId) {
+        return store.getLastUpdated(nodeId);
+    }
+
     @Override
     public ControllerNode addNode(NodeId nodeId, IpAddress ip, int tcpPort) {
         checkNotNull(nodeId, INSTANCE_ID_NULL);