R1:251ee9b660b8

Add dedicated "reviewers" storage to Differential and do double writes

Summary:
Ref T10967. This is an incremental step toward removing "reviewers" back to a dedicated storage table so we can handle changes like T11050.

This adds the storage table, and starts doing double writes to it (so new or updated reviewers write to both the old edge table and the new "reviewers" table).

Then we can do a migration, swap readers over one at a time, and eventually remove the old write and old storage and then…