Fixed #6791: added a write-through cache session backend: session data is...

Fixed #6791: added a write-through cache session backend: session data is written through the cache to the database, but read from the cache for speed. Thanks to jhenry, mcroydon, and jdunck.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9727 bcc190cf-cafb-0310-a4f2-bffc1f526a37