This avoids the problem of, for example, saving a change to the Site model in
the admin interface and then seeing the wrong instanec returned in the next
call to get_current_site().

It's still possible to end up with an inconsistent cache if update() is used to
change the Site model, but that's pretty unavoidable. It's also a slightly odd
way to update a Site model, so if you really need to do that, you can manage to
call SiteManager.clear() at the same time.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9908 bcc190cf-cafb-0310-a4f2-bffc1f526a37