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
pools = models.Pool.slots_stats(session=session)
        for pool_name, slot_stats in pools.items():
            Stats.gauge(f'pool.open_slots.{pool_name}', slot_stats["open"])
            Stats.gauge(f'pool.queued_slots.{pool_name}', slot_stats[State.QUEUED])
            Stats.gauge(f'pool.running_slots.{pool_name}', slot_stats[State.RUNNING])
    @provide_session
    def heartbeat_callback(self, session: Session = None) -> None:
        Stats.incr('scheduler_heartbeat', 1, 1)
    @provide_session
    def adopt_or_reset_orphaned_tasks(self, session: Session = None):
        """
        Reset any TaskInstance still in QUEUED or SCHEDULED states that were
        enqueued by a SchedulerJob that is no longer running.
        :return: the number of TIs reset
        :rtype: int
        """
        timeout = conf.getint('scheduler', 'scheduler_health_check_threshold')
        num_failed = session.query(SchedulerJob).filter(
            SchedulerJob.state == State.RUNNING,
            SchedulerJob.latest_heartbeat < (timezone.utcnow() - timedelta(seconds=timeout))
        ).update({"state": State.FAILED})
        if num_failed:
            self.log.info("Marked %d SchedulerJob instances as failed", num_failed)
            Stats.incr(self.__class__.__name__.lower() + '_end', num_failed)
        resettable_states = [State.SCHEDULED, State.QUEUED, State.RUNNING]
        query = (
            session.query(TI).filter(TI.state.in_(resettable_states))