added toString

Change-Id: I5ea98752b3f753eb398c83616afae11305d84e72

@@ -6,6 +6,8 @@ import java.util.Objects;
 
 import org.onlab.onos.store.Timestamp;
 
+import com.google.common.base.MoreObjects;
+
 /**
  * Wrapper class to store Timestamped value.
  * @param <T>
@@ -70,6 +72,14 @@ public final class Timestamped<T> {
         return Objects.equals(this.timestamp, that.timestamp);
     }
 
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(getClass())
+                    .add("timestamp", timestamp)
+                    .add("value", value)
+                    .toString();
+    }
+
     // Default constructor for serialization
     @Deprecated
     protected Timestamped() {