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
:param file_path: the path to the Python file that should be executed
        :type file_path: str
        :param callback_requests: failure callback to execute
        :type callback_requests: List[airflow.utils.dag_processing.CallbackRequest]
        :param pickle_dags: whether serialize the DAGs found in the file and
            save them to the db
        :type pickle_dags: bool
        :param session: Sqlalchemy ORM Session
        :type session: Session
        :return: number of dags found, count of import errors
        :rtype: Tuple[int, int]
        """
        self.log.info("Processing file %s for tasks to queue", file_path)
        try:
            dagbag = DagBag(file_path, include_examples=False, include_smart_sensor=False)
        except Exception:  
            self.log.exception("Failed at reloading the DAG file %s", file_path)
            Stats.incr('dag_file_refresh_error', 1, 1)
            return 0, 0
        if len(dagbag.dags) > 0:
            self.log.info("DAG(s) %s retrieved from %s", dagbag.dags.keys(), file_path)
        else:
            self.log.warning("No viable dags retrieved from %s", file_path)
            self.update_import_errors(session, dagbag)
            return 0, len(dagbag.import_errors)
        self.execute_callbacks(dagbag, callback_requests)
        
        dagbag.read_dags_from_db = True