Backup/restore: PITR surface, point-in-time recovery reported as unsupported #745

The PITR surface of design section 6 (#396, docs/design/15-backup-restore.md): UpdateContinuousBackups, DescribeContinuousBackups and RestoreTableToPointInTime answer as the design states, with TableNotFoundException for a missing table on all four backends; the catalog migrations that drop the stale PITR state (PostgreSQL migration 004, SQLite schema bump, Cassandra V006, MongoDB's continuous_backups collection). Smaller than its first draft because #384 already removed the trait's restore_table_to_point_in_time. #446 describes the current behaviour.

Part of #624. The design's PR 1. Implemented in #335, which needs a rebase onto main.