Cache: multiple variants of a resource now can be stored.

Cache: multiple variants of a resource now can be stored.

If a variant stored can't be used to respond to a request, the variant
hash is used as a secondary key.

Additionally, if we previously switched to a secondary key, while storing
a response to cache we check if the variant hash still apply.  If not, we
switch back to the original key, to handle cases when Vary changes.