[1.2.X] Fixed #14433 - replaced a thread-unsafe solution to #10235 introduced in [13980]

This patch also addresses sitemap code found in contrib/gis, which [13980]
did not.

Thanks to gabrielhurley for the initial patch.

Refs #10235, #14386

Backport of [14141] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14142 bcc190cf-cafb-0310-a4f2-bffc1f526a37