Backup/restore: [backup.store] configuration, the store identity file, backup purge #670

The [backup.store] configuration with its per-backend defaults and validation, put_if_absent on the store trait, the store identity file and the /health component, the maintenance claim on every backend, DeleteBackup against the configured store, and extenddb backup list and backup purge (design sections 4(c), 4(f), 7, 9, PR 7).

Part of #624. Implemented in #672.