Return MergedList<T> from list_all_* and drop the _with_cache_hint variants #1430

#1429 (for #1411) adds Peer::list_all_*_with_cache_hint() methods next to the existing list_all_* methods. They return a MergedList<T> { items, ttl_ms, cache_scope } so callers can cache a merged list. The new methods sit beside the old ones only because changing the return type of list_all_* is a breaking change.

In the next major release, list_all_* could return MergedList<T> directly, and the five _with_cache_hint methods would be removed. That leaves one method per list kind instead of two. Callers that only need the items would write .items.

This is a breaking change, so it should ship with the other changes in the next major release rather than in a major release of its own.