When logging in, change the session key whilst preserving any existing

sesssion. This means the user will see their session preserved across a login
boundary, but somebody snooping the anonymous session key won't be able to view
the authenticated session data.

This is the final piece of the session key handling changes.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8459 bcc190cf-cafb-0310-a4f2-bffc1f526a37