R1:d15fb20fe6a9

Support storage of Differential hunk data in Files

Summary:
Ref T12932. For long-lived installs, one of the largest tables tends to be the hunk data table. Although it doesn't grow tremendously fast, it's also well suited to storage in Files instead of the database (infrequent access, relatively large blobs of data, mostly one-at-a-time access), and earlier work anticipated eventually adding support for Files storage.

Make Files storage work, and provide `bin/differential migrate-hunk` to manually…