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/controllers/notes_controller.rb

@@ -6,7 +6,7 @@ class NotesController < ApplicationController
   end
 
   def index
-    @notes = Note.search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @notes = Note.includes(:creator).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
     respond_with(@notes) do |format|
       format.html { @notes = @notes.includes(:creator) }
       format.xml do