Fixed #3615: Added support for loading fixtures with forward references on...

Fixed #3615: Added support for loading fixtures with forward references on database backends (such as MySQL/InnoDB) that do not support deferred constraint checking. Many thanks to jsdalton for coming up with a clever solution to this long-standing issue, and to jacob, ramiro, graham_king, and russellm for review/testing. (Apologies if I missed anyone else who helped here.)

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16590 bcc190cf-cafb-0310-a4f2-bffc1f526a37