R1:39afc0f97cfc

Add an AES256 storage format for at-rest encryption

Summary:
Ref T11140. This doesn't do anything yet since there's no way to enable it and no way to store master keys.

Those are slightly tougher problems and I'm not totally satisfied that I have an approach I really like for either problem, so I may wait for a bit before tackling them. Once they're solved, this does the mechanical encrypt/decrypt stuff, though.

This design is substantially similar to the AWS S3 server-side encryption design, and…