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.

docs/internals/deprecation.txt

@@ -479,6 +479,8 @@ these changes.
 * The ``zh-cn`` and ``zh-tw`` language codes will be removed and have been
   replaced by the ``zh-hans`` and ``zh-hant`` language code respectively.
 
+* The internal ``django.utils.functional.memoize`` will be removed.
+
 2.0
 ---