Avoid slowing down AR object initialization

2d73f5a forces AR to enter the `define_attribute_methods` method whenever it
instantiate a record from the `init_with` entry point. This is a potential
performance hotspot, because `init_with` is called from all `find*` family
methods, and `define_attribute_methods` is slow because it tries to acquire
a lock on the mutex everytime it is entered.

By using [DCL](http://en.wikipedia.org/wiki/Double-checked_locking), we can
avoid grabbing the lock most of the time when the attribute methods are already
defined (the common case). This is made possible by the fact that reading an
instance variable is an atomic operation in Ruby.

Credit goes to Aaron Patterson for pointing me to DCL and filling me in on the
atomicity guarantees in Ruby.

[*Godfrey Chan*, *Aaron Patterson*]

activerecord/lib/active_record/attribute_methods.rb

@@ -66,6 +66,7 @@ module ActiveRecord
       # Generates all the attribute related methods for columns in the database
       # accessors, mutators and query methods.
       def define_attribute_methods # :nodoc:
+        return false if @attribute_methods_generated
         # Use a mutex; we don't want two thread simultaneously trying to define
         # attribute methods.
         generated_attribute_methods.synchronize do