If we have no consensus, save any well-signed consensus we're given?

When starting with its clock "a bit in the future" with no existing consensus, Arti will fetch the latest consenus... then throw it away because it is not new enough, and try it again.

That's silly: even if the consensus is not usable, it is better than no consensus at all, since we could save it and use it for applying consensus diffs. It would also help us pick a better If-Modified-Since for future consensus requests.

Found while investigating #329 (closed). See also #401.