BUG 1595 - Clustering : NPE on startup

This seemed to be happening because of recovery where the topology manager
was trying to add something into the datastore which was already present in
the journal.

The fix was to add a timestamp to each modification so that they do not
appear to be the same even if their content was exactly the same.

Change-Id: I2d5c98bd87ded7c8b64b8cebf757bfa073327061
Signed-off-by: Moiz Raja <moraja@cisco.com>