Changed the way cache specifiers are parsed.

Allows us to reuse the same code in multiple places, avoiding new and
interesting bugs (the testing framework had a DIY version that was slightly
wrong, for example). Fixed #9833.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9936 bcc190cf-cafb-0310-a4f2-bffc1f526a37