fixes task cleanup when FK on TaskState.task_id
fixes task cleanup when FK on TaskState.task_id

fix from issue #358. uses django's managers to cleanup
axiom-data-science/django-celery,axiom-data-science/django-celery,kanemra/django-celery,kanemra/django-celery,axiom-data-science/django-celery,celery/django-celery,celery/django-celery,kanemra/django-celery,celery/django-celery