Backup/restore: backup sync and its options, migrate-legacy #746

The catalog-loss recovery and maintenance commands of design section 4 (#396): extenddb backup sync with store-root scoping, the store id, the maintenance claim, the tombstone and identity checks and the insert-if-absent; --prune-missing, --forget-store, --forget-backup, --move-store (with list_backup_rows_by_root and rewrite_store) and --adopt-store; the SQLite serve lock on every command that writes and the dead-owner resolutions such a command runs first; extenddb backup migrate-legacy <arn>|--all with its re-read, validation and count checks and its refusals. backup remove-target shipped with PR 9 (#740). The design's PR 10 section carries the acceptance list.

Part of #624. The design's PR 10. Depends on PR 6 (#671) and PR 8 (#700).