If you have a DAG with `max_active_runs=1` and it is running some tasks, and then you manually trigger another run of this DAG, both runs end up not scheduling anything and you see this message over and over in the logs:

```
[2020-10-16 14:00:34,118]  2437572 {{airflow.jobs.scheduler_job.SchedulerJob scheduler_job.py:1657}} INFO - DAG scenario1_case2_01_1 already has 1 active runs, not queuing any more tasks
```

A longer term fix for this would be to introduce a "queued" state for DagRuns, and then when manually creating dag runs set it to queued, and only have the scheduler set DagRuns to running.

That may or may not be less work than fixing the current mechanism.
airflow/jobs/scheduler_job.py
"""
        Is this SchedulerJob alive?
        We define alive as in a state of running and a heartbeat within the
        threshold defined in the ``scheduler_health_check_threshold`` config
        setting.
        ``grace_multiplier`` is accepted for compatibility with the parent class.
        :rtype: boolean
        """
        if grace_multiplier is not None:
            
            return super().is_alive(grace_multiplier=grace_multiplier)
        scheduler_health_check_threshold: int = conf.getint('scheduler', 'scheduler_health_check_threshold')
        return (
            self.state == State.RUNNING and
            (timezone.utcnow() - self.latest_heartbeat).total_seconds() < scheduler_health_check_threshold
        )
    @provide_session
    def _change_state_for_tis_without_dagrun(
        self,
        old_states: List[str],
        new_state: str,
        session: Session = None
    ) -> None:
        """
        For all DAG IDs in the DagBag, look for task instances in the