Added a WallClockTimestamp and associated interface/implementation for generating it. This will be used by the initial implementation of distributed host store
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/impl/MastershipBasedTimestamp.java b/core/store/dist/src/main/java/org/onlab/onos/store/impl/MastershipBasedTimestamp.java
index e6317bb..9721024a 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/impl/MastershipBasedTimestamp.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/impl/MastershipBasedTimestamp.java
@@ -10,8 +10,12 @@
import com.google.common.collect.ComparisonChain;
/**
- * Default implementation of Timestamp.
- * TODO: Better documentation.
+ * A logical timestamp that derives its value from two things:
+ * <ul>
+ * <li> The current mastership term of the device.</li>
+ * <li> The value of the counter used for tracking topology events observed from
+ * the device during that current time of a device. </li>
+ * </ul>
*/
public final class MastershipBasedTimestamp implements Timestamp {