[wip]Making swarm query node always return checkin

@hbeeken , as discussed with @knolleary today, the query nodes will always have to return something. Therefore I modified the Swarm query node to ensure that its behaviour is consistent with what I implemented in the Instagram node.

Started working on this too as @hbeeken is out of office until Wednesday. Also did some tidy-up whilst I was there.

I expect the unit tests to fail (although the node looks to be working OK) so hence the WIP tag until I get those working.
The node always did this. The query node always returned the JSON of the latest check-in <b>since the node was initialized</b> regardless of whether its been returned before. 

    /**
     * Swarm query node - will return the most recent check-in since
     * the node has been initialized. If a check-in exists the node will always return 
     * the most recent even if no new check-ins have happened since the previous query.
     * The node only populates msg.payload when a check-in is found.  
     */


The question I believe is which of the following is the desired behaviour:

(1) To only consider check-ins since the node has been initialized
(2) To return the latest check-in even if this was before the node was initialized

Currently (1) is implemented, (2) is in @zobalogh pull request.