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/views/posts/show.html.erb

@@ -120,7 +120,7 @@
         <% if !CurrentUser.user.is_builder? %>
           <h2>Before commenting, read the <%= link_to "how to comment guide", wiki_pages_path(:search => {:title => "howto:comment"}) %>.</h2>
         <% end %>
-        <%= render "comments/partials/index/list", :comments => @post.comments.visible(CurrentUser.user), :post => @post, :show_header => false %>
+        <%= render "comments/partials/index/list", :comments => @post.comments.visible(CurrentUser.user).includes(:creator), :post => @post, :show_header => false %>
       </section>
 
       <section id="notes" style="display: none;">