Fixed #6099: the filebased cache backend now uses md5 hashes of keys instead...

Fixed #6099: the filebased cache backend now uses md5 hashes of keys instead of sanitized filenames. For good measure, keys are partitioned into subdirectories using the first few bits of the hash. Thanks, sherbang.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6887 bcc190cf-cafb-0310-a4f2-bffc1f526a37