scan_items/scan_values: accept wrapped stores now that items()/values() are batched #6

After #4's PR, store.items()/values() are correct and batched (BatchGetItem, 100 keys per call) through any dol wrapper. scan_items/scan_values still refuse wrapped stores with a TypeError (existing test test_scan_refuses_wrapped_store); they could delegate to items()/values() for wrapped stores and keep the single-scan path for bare ones. Left as is because it changes a tested public behaviour (relaxing a refusal). Recommendation: do it.