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
timer.stop(send=False)
                if is_lock_not_available_error(error=e):
                    self.log.debug("Critical section lock held by another Scheduler")
                    Stats.incr('scheduler.critical_section_busy')
                    session.rollback()
                    return 0
                raise
            return num_queued_tis
    def _create_dag_runs(self, dag_models: Iterable[DagModel], session: Session) -> None:
        """
        Unconditionally create a DAG run for the given DAG, and update the dag_model's fields to control
        if/when the next DAGRun should be created
        """
        for dag_model in dag_models:
            dag = self.dagbag.get_dag(dag_model.dag_id, session=session)
            dag_hash = self.dagbag.dags_hash.get(dag.dag_id)
            dag.create_dagrun(
                run_type=DagRunType.SCHEDULED,
                execution_date=dag_model.next_dagrun,
                start_date=timezone.utcnow(),
                state=State.RUNNING,
                external_trigger=False,
                session=session,
                dag_hash=dag_hash,
                creating_job_id=self.id,
            )
        self._update_dag_next_dagruns(dag_models, session)
        
        
    def _update_dag_next_dagruns(self, dag_models: Iterable[DagModel], session: Session) -> None: