Fixed #21351 -- Replaced memoize with Python's lru_cache.

Replaced the custom, untested memoize with a similar decorator from Python's
3.2 stdlib. Although some minor performance degradation (see ticket), it is
expected that in the long run lru_cache will outperform memoize once it is
implemented in C.

Thanks to EvilDMP for the report and Baptiste Mispelon for the idea of
replacing memoize with lru_cache.

django/utils/functional.py

@@ -2,6 +2,7 @@ import copy
 import operator
 from functools import wraps
 import sys
+import warnings
 
 from django.utils import six
 from django.utils.six.moves import copyreg
@@ -24,6 +25,10 @@ def memoize(func, cache, num_args):
 
     Only the first num_args are considered when creating the key.
     """
+    warnings.warn(u"memoize wrapper is deprecated and will be removed in "
+                  u"Django 1.9. Use django.utils.lru_cache instead.",
+                  PendingDeprecationWarning, 2)
+
     @wraps(func)
     def wrapper(*args):
         mem_args = args[:num_args]