Added guaranteed atomic creation of new session objects. Slightly backwards

incompatible for custom session backends.

Whilst we were in the neighbourhood, use a larger range of session key values
to save a small amount of time and use the hardware-base random numbers where
available (transparently falls back to pseudo-RNG otherwise).

Fixed #1080


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8340 bcc190cf-cafb-0310-a4f2-bffc1f526a37