[1.2.X] Fixed #13946 -- Modified the database cache backend to use the...

[1.2.X] Fixed #13946 -- Modified the database cache backend to use the database router to determine availability of the cache table. Thanks to tiemonster for the report.

Backport of r13473 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13474 bcc190cf-cafb-0310-a4f2-bffc1f526a37