Add a new origin_visit_get_latest endpoint.
snapshot_get_latest did two things: find the latest origin, and get its snapshot. Now, it only calls origin_visit_get_latest, then snapshot_get.
snapshot_get_latest should also be deprecated, because it only fetches some of the branches (for large snapshots).
Finally, with the new data model (types are associated to visits, not origins), origin_visit_get_latest is required to know the type of the loader that got the snapshot (needed in swh-indexer).
Also, note that origin_visit_get_latest only supports URLs as argument, not origin ids. Since we are migrating away from origin ids, I don't think it makes sense to add support for origin ids in a new endpoint.
Migrated from D1581 (view on Phabricator)