Move archive cache responsibility to mirror source.

This will require at least this version of Flapjack on both sides for the mirror process to work properly.

See #653 and previous PR #661, which didn't really solve the issue properly.
could we make 't' and 'f' a little more expressive? eg  'nonempty' and 'empty' or some such? (assuming I've understood what's going on, which I'm not too sure about)