R1:5b1262c98b8f

Add a `bin/hunks` script to manage migrations of hunk data

Summary:
Ref T4045. Ref T5179. While we'll eventually need to force a migration, we can let installs (particularly large installs) do an online migration for now. This moves hunks to the new storage format one at a time.

(Note that nothing writes to the new store yet, so this is the only way to populate it.)

WARNING: Installs, don't run this yet! It won't compress the data. Wait until it can also do compression.

Test Plan: Added a `break;`…