internal/v4: implement changes/published

As recently added to the specification.

If you're going to do that, the client can always specify a limit of 200...
There may be an issue, of course, if there are enough charms to
cause memory problems when all their ids and times are returned
to the client. We need to think some more about possible streaming
APIs for large results. I'm not that keen on the offset approach, as
it can result in missing/inconsistent data when the set of results
changes between the two requests.

For this particular request, the client can also limit things by specifying
the start date - I think it will be a while before the number of charms/bundles
published on a single day poses a memory problem.