rdf.module triggers a user_load() per node or comment author

Now that we have user_load_multiple(), it'd be much better to preload all the user accounts which are node or comment authors in rdf_node_load() and rdf_comment_load() (or possibly user_comment_load() and user_node_load()).

However, we can't do that 100% safely until #636992: Entity loading needs protection from infinite recursion is in, and ideally it'd be in a stage separate from raw loading, since doing it in node_load() will cause problems for http://drupal.org/project/entitycache