Django denial of service via empty session record creation
`contrib.sessions.middleware.SessionMiddleware` in Django 1.8.x before 1.8.4, 1.7.x before 1.7.10, 1.4.x before 1.4.22, and possibly other versions allows remote attackers to cause a denial of service (session store consumption or session record removal) via a large number of requests to `contrib.auth.views.logout`, which triggers the creation of an empty session record.