Perf: save ~9% of object allocations on heavy requests.

The per-thread registry is keyed on the class name, and each request for
the class name returns a new string. This is in the hot path for a lot
of Active Record behavior, so we easily accumulate thousands of repeated
strings.

To fix, we simply cache the key when the class is first extended with
the module.

TODO: Eliminate this module. The per-thread instance concept is common,
but this technique confuses and obfuscates.

activesupport/lib/active_support/per_thread_registry.rb

@@ -32,12 +32,15 @@ module ActiveSupport
   #
   # If the class has an initializer, it must accept no arguments.
   module PerThreadRegistry
+    def self.extended(object)
+      object.instance_variable_set '@per_thread_registry_key', object.name.freeze
+    end
+
     def instance
-      Thread.current[name] ||= new
+      Thread.current[@per_thread_registry_key] ||= new
     end
 
     protected
-
       def method_missing(name, *args, &block) # :nodoc:
         # Caches the method definition as a singleton method of the receiver.
         define_singleton_method(name) do |*a, &b|