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.

test/unit/post_test.rb

@@ -1793,7 +1793,7 @@ class PostTest < ActiveSupport::TestCase
         post.uploader_id = user2.id
         assert_equal(user2.id, post.uploader_id)
         assert_equal(user2.id, post.uploader_id)
-        assert_equal(user2.name, post.uploader_name)
+        assert_equal(user2.name, post.uploader.name)
       end
 
       context "tag post counts" do