Fixed #11555 -- Made SessionBase.session_key read-only. Cleaned up code slightly. Refs #13478.

      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