[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.
I've been thinking about this some more....I'm not entirely sure why the only change required isn't an else statement here to say:

                  } else {
                      msg.payload = null; // or should it be {}?
                      callback(msg);
                  }            

Why are we changing the behaviour of the node to always return the latest check-in even if that check-in took place before the node was in the flow? I'm not 100% clear on the requirements. If it's just to always return something then msg.payload = null (or is it {}?) is enough at this line (line 122). If it makes sense to change the functionality of the node then that's fine too but was that part of the requirements? If it is then all the node documentation needs updating too (in the swarm-html docs) which I don't believe has been done.