[1.0.X] 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.

Backport of r9936 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9937 bcc190cf-cafb-0310-a4f2-bffc1f526a37