internal/v4: implement debug/status endpoint

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

It looks like charmstore.StartTime in the code, which i think is reasonable.
Also, I deliberately don't want it to be necessarily always the server
executable start time - I guess it might be possible that some executable
might be running for ages and then suddenly decide to start the charm store...

Perhaps that's a silly thing to contemplate though and maybe this variable would be best off declared locally inside v4 in fact.