Backup/restore: backup sync, --adopt-store, --prune-missing, the sqlite-memory startup sync #746

The catalog-loss recovery of design section 4(g) (#396): extenddb backup sync lists the configured store's root, checks each <account>/<table>/<backup_id>/ prefix against the catalog, verifies the manifest, its sidecar and its data files, and inserts an AVAILABLE row with an insert-if-absent (BackupCatalog::insert_backup_row on all four backends). --prune-missing moves an AVAILABLE row whose manifest is gone to DELETING. --adopt-store takes over a root whose identity file names another catalog (4(c)). A sqlite-memory server adopts and syncs its previous run's root at startup. The cross-backend matrix replaces its hand-seeded backup_index row with the command. The SQLite serve lock and the maintenance claim apply as for backup purge.

The design's PR 10 is split into three PRs; this is the first. --move-store, --forget-store and --forget-backup follow in #820, and migrate-legacy in #821. backup remove-target shipped with PR 9 (#740).

Part of #624. Depends on PR 9 (#740).