Offline key caching

A customer goes out to a field office, that is off the grid. They want to be able to encrypt things with a key, but they can't talk to KMS. The idea would be they could fill up some cache of keys prior to going offline, then when they need to encrypt data, they can use those keys.

This sounds like a special case of the general pattern for the data-key caching redesign, with the additional request of the ability to pre-warm entries into a cache.