Backup/restore: CreateBackup job, worker and reconciler duties #699

The store-backed CreateBackup job of design section 5.1 (#396) on PostgreSQL, SQLite and MongoDB: the snapshot in the request, the claim, the pipelined validate/compress/upload, the heartbeat and failure path, the byte-budget paging on the PostgreSQL and SQLite snapshot streams, the item validator, and the reconciler duties of section 4 on backup rows (dead CREATING rows resolved, FAILED and DELETED prefixes, tombstone retention). Depends on PR 6 (#671) and PR 7 (#672).

Part of #624. Implemented in #700.