Change the conditions for a data-set being out of date

- If it's never been updated, we can assume that it's in work. This can be subject to discussion, but my hunch is this is going to be the simplest way to maintain this
- Return full data-set status objects for both in-date and out-of-date data-sets. We can mod the status endpoint to always return both, or just one, or just the other, depending on need. Again, a hunch, but I feel we should be able to see how in-date something is as well as how out-of-date it is.
- Add `max-age-expected` to the status object