Implemented a flush() method on sessions that cleans out the session and

      regenerates the key. Used to ensure the caller gets a fresh session at logout,
for example.

Based on a patch from mrts. Refs #7515.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8342 bcc190cf-cafb-0310-a4f2-bffc1f526a37