users: drop id_to_name, name_to_id caching.

Changes:

* Drop Users.id_to_name.
* Don't cache Users.name_to_id.
* Replace calls to name_to_id with find_by_name when possible.
* Don't autodefine creator_name in belongs_to_creator.
* Don't autodefine updater_name in belongs_to_updater.
* Instead manually define creator_name / updater_name only on models that need
  to return these fields in the api.

id_to_name was cached to reduce the impact of N+1 query patterns in
certain places, especially in api responses that return creator_name /
updater_name fields. But it still meant we were doing N calls to
memcache. Using `includes` to prefetch users avoids this N+1 pattern.

name_to_id had no need be cached, it was never used in any performance-
sensitive contexts.

Avoiding caching also avoids the need to keep these caches consistent.

app/logical/post_sets/post.rb

@@ -135,6 +135,10 @@ module PostSets
         else
           temp = ::Post.tag_match(tag_string).where("true /* PostSets::Post#posts:2 */").paginate(page, :count => post_count, :limit => per_page)
         end
+
+        # XXX HACK: uploader_name is needed in api responses and in data-uploader attribs (visible to mods only).
+        temp = temp.includes(:uploader) if format.to_sym != :html || CurrentUser.is_moderator?
+
         temp.each # hack to force rails to eager load
         temp
       end