Use Owner role to help filling user_set: owners must be users, never
groups.
Only look for users among group_ids when they are candidate for indexation
into a catalog_role column. This should very significantly reduce the
number of user lookups, even bringing it to zero on instance with default
catalog setting of catalog_role_set = set(["viewable_owner"]) .
Also, only look for users if there are candidates for lookup.
Also, rename local variable to better describe its content.
Also, use set.__iadd__ instead of set.update (shorter).