Revert some ActiveSupport::Callbacks changes.

This reverts commits 911a0859ac and
30b31f51af.

Reason: these changes make the Active Model tests fail randomly.

Some examples:
http://travis-ci.org/#!/rails/rails/jobs/1498992
http://travis-ci.org/#!/rails/rails/jobs/1496948
http://travis-ci.org/#!/rails/rails/jobs/1489985

This script was used to reproduce these breaks:
https://gist.github.com/f6828a03ee4d40bffbc3

200 times, 0 failures

activesupport/lib/active_support/callbacks.rb

@@ -328,17 +328,26 @@ module ActiveSupport
       # if it was not yet defined.
       # This generated method plays caching role.
       def __define_callbacks(kind, object) #:nodoc:
-        chain = object.send("_#{kind}_callbacks")
-        name = "_run_callbacks_#{chain.object_id.abs}"
+        name = __callback_runner_name(kind)
         unless object.respond_to?(name, true)
+          str = object.send("_#{kind}_callbacks").compile
           class_eval <<-RUBY_EVAL, __FILE__, __LINE__ + 1
-            def #{name}() #{chain.compile} end
+            def #{name}() #{str} end
             protected :#{name}
           RUBY_EVAL
         end
         name
       end
 
+      def __reset_runner(symbol)
+        name = __callback_runner_name(symbol)
+        undef_method(name) if method_defined?(name)
+      end
+
+      def __callback_runner_name(kind)
+        "_run__#{self.name.hash.abs}__#{kind}__callbacks"
+      end
+
       # This is used internally to append, prepend and skip callbacks to the
       # CallbackChain.
       #
@@ -350,6 +359,7 @@ module ActiveSupport
         ([self] + ActiveSupport::DescendantsTracker.descendants(self)).reverse.each do |target|
           chain = target.send("_#{name}_callbacks")
           yield target, chain.dup, type, filters, options
+          target.__reset_runner(name)
         end
       end
 
@@ -437,9 +447,12 @@ module ActiveSupport
           chain = target.send("_#{symbol}_callbacks").dup
           callbacks.each { |c| chain.delete(c) }
           target.send("_#{symbol}_callbacks=", chain)
+          target.__reset_runner(symbol)
         end
 
         self.send("_#{symbol}_callbacks=", callbacks.dup.clear)
+
+        __reset_runner(symbol)
       end
 
       # Define sets of events in the object lifecycle that support callbacks.