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
old_states and set them to new_state if the corresponding DagRun
        does not exist or exists but is not in the running state. This
        normally should not happen, but it can if the state of DagRuns are
        changed manually.
        :param old_states: examine TaskInstances in this state
        :type old_states: list[airflow.utils.state.State]
        :param new_state: set TaskInstances to this state
        :type new_state: airflow.utils.state.State
        """
        tis_changed = 0
        query = session \
            .query(models.TaskInstance) \
            .outerjoin(models.TaskInstance.dag_run) \
            .filter(models.TaskInstance.dag_id.in_(list(self.dagbag.dag_ids))) \
            .filter(models.TaskInstance.state.in_(old_states)) \
            .filter(or_(
                
                models.DagRun.state != State.RUNNING,
                models.DagRun.state.is_(None)))  
        
        
        if self.using_sqlite or self.using_mysql:
            tis_to_change: List[TI] = with_row_locks(query, of=TI,
                                                     **skip_locked(session=session)).all()
            for ti in tis_to_change:
                ti.set_state(new_state, session=session)
                tis_changed += 1
        else:
            subq = query.subquery()
            current_time = timezone.utcnow()