title: Airflow cleared backfill tasks do not get picked by the scheduler
body: <p>When a task in a successful backfill run is cleared DagRun with running state is created but it is not picked by the scheduler. Is this the expected behaviour?</p>
  
  <p>If there are many of such DagRuns and the backfill command is executed, all the DagRuns will be ran at the same time, not following the max_active_runs parameter.</p>
  
  <p>The only workaround I've found until now is to clear the tasks, then mark the dagruns (not the task instances) as successful via UI, and then run the backfill.</p>