R1:0aa913805d14

Add an alternate "modern" hunk datastore

Summary:
Ref T4045. Ref T5179. Hunk storage has two major issues:

- It's utf8, but actual diffs are binary.
- It's huge and can't be compressed or archived.

This introduces a second datastore which solves these problems: by recording hunk encoding, supporting compression, and supporting alternate storage. There's no actual compression or storage support yet, but there's space in the table for them.

Since nothing actually uses hunk IDs, it's fine to have…