Backup/restore: catalog index, BackupCatalog and JobClaim on every backend, the legacy reader #669

The catalog index of design section 4 (backup_index, the restore job record) on PostgreSQL, SQLite, MongoDB and Cassandra, the BackupCatalog and JobClaim traits the engine binds, the reconciler, the [backup] section, and the in-catalog paths of #384 kept as a frozen LegacyBackupReader. Catalog migration id 5 (stacked on #666).

Part of #624. Implemented in #671.