Rewrite %file table structure

Add `attrs` field for backend state information (multi-part migration,
multi-part upload, etc). Change `size` to be an integer rather than text
field. (Storage space will be about the same, but int makes more sense).
Size the `key` and `signature` for ~512 bits. The base64 encoding allows
6-bits per char. At 86 chars, we have 516 bits maximum space.