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.