[1.0.X] Corrected a problem with the database cache backend, and refactored...

[1.0.X] Corrected a problem with the database cache backend, and refactored the cache test suite to ensure that all the backends are actually tested.

Partial merge of r10031 and r10071 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10086 bcc190cf-cafb-0310-a4f2-bffc1f526a37