Super cache

I am going to enable Shielding on our edge cache. To do that I need cache headers to be as simple as possible.

`s-max-age` is only really useful if it has a different value to `max-age` (it overrides `max-age` on a proxy cache). They never differ for us, so it is going.

The 2 `stale` headers are not ratified and, in particular, do not work on our edge cache (you configure those values) so they are going too.