[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.
Ah ok now I understand :-) Thinking further, considering any change to a node configuration means that nodes are redeployed it does make sense to return the most recent checkin regardless of when the node was deployed. Thanks for clarifying.

In which case this pull request should be a simple change to not use the "afterTimestamp" for the query node and set 'msg.payload = null' if no checkins at all have ever been made. The various node docs should also be updated.