Fixed #12540, #12541 -- Added database routers, allowing for configurable...

Fixed #12540, #12541 -- Added database routers, allowing for configurable database use behavior in a multi-db setup, and improved error checking for cross-database joins.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12272 bcc190cf-cafb-0310-a4f2-bffc1f526a37