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/artist_versions_controller.rb

@@ -2,7 +2,7 @@ class ArtistVersionsController < ApplicationController
   respond_to :html, :xml, :json
 
   def index
-    @artist_versions = ArtistVersion.search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @artist_versions = ArtistVersion.includes(:updater).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
     respond_with(@artist_versions) do |format|
       format.xml do
         render :xml => @artist_versions.to_xml(:root => "artist-versions")

app/controllers/comments_controller.rb

@@ -83,7 +83,7 @@ private
   end
 
   def index_by_comment
-    @comments = Comment.search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @comments = Comment.includes(:creator, :updater).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
     respond_with(@comments) do |format|
       format.atom do
         @comments = @comments.includes(:post, :creator).load

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

app/controllers/pools_controller.rb

@@ -17,7 +17,7 @@ class PoolsController < ApplicationController
   end
 
   def index
-    @pools = Pool.search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @pools = Pool.includes(:creator).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
     respond_with(@pools) do |format|
       format.xml do
         render :xml => @pools.to_xml(:root => "pools")

app/controllers/wiki_pages_controller.rb

@@ -15,7 +15,7 @@ class WikiPagesController < ApplicationController
   end
 
   def index
-    @wiki_pages = WikiPage.search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @wiki_pages = WikiPage.includes(:creator).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
     respond_with(@wiki_pages) do |format|
       format.html do
         if params[:page].nil? || params[:page].to_i == 1