Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.
Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.
* Prevented stale session files from being loaded
* Added removal of stale session files in django-admin.py clearsessions
Thanks ej for the report, crodjer and Elvard for their inputs.
This also removes the implicit initialization of the session key on the first access in favor of explicit initialization.
git-svn-id: http://code.djangoproject.com/svn/django/trunk@17155 bcc190cf-cafb-0310-a4f2-bffc1f526a37