Backup/restore: RestoreTableFromBackup job from the store and the cross-backend matrix #739

The store-backed restore job of design section 5.3 (#396) into every backend (PostgreSQL, SQLite, MongoDB, Cassandra): the request's registration under the backup row's lock, the download and spool with verification before any load, bulk_load with progress per backend, the ACTIVE flip, the reconciler's resume of a dead restore, and the twelve-cell cross-backend matrix with its nightly workflow (the design's PR 9a and PR 9b as one PR). Depends on PR 8 (#700).

Part of #624.

Implemented in #740.