internal/v4: implement debug/status endpoint

As newly documented in the charm store.
Not all entries are implemented yet.

Can this be written in a mongo side run aggregate?

I don't know mongo well enough, but I know in SQL, I'd much prefer a

    SELECT COUNT() FROM Entities WHERE URLSeries='bundle'
and
    SELECT COUNT() FROM Entities WHERE URLSeries!='bundle'
and
    SELECT COUNT() FROM Entities WHERE URLUser=""

3 queries which can utilize indexes rather than iterating the entire table.