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
for attempt in tenacity.Retrying(
            retry=tenacity.retry_if_exception_type(exception_types=OperationalError),
            wait=tenacity.wait_random_exponential(multiplier=0.5, max=5),
            stop=tenacity.stop_after_attempt(settings.MAX_DB_RETRIES),
            before_sleep=tenacity.before_sleep_log(self.log, logging.DEBUG),
            reraise=True
        ):
            with attempt:
                self.log.debug(
                    "Running dagbag.sync_to_db with retries. Try %d of %d",
                    attempt.retry_state.attempt_number,
                    settings.MAX_DB_RETRIES
                )
                dagbag.sync_to_db()
        if pickle_dags:
            paused_dag_ids = DagModel.get_paused_dag_ids(dag_ids=dagbag.dag_ids)
            unpaused_dags: List[DAG] = [
                dag for dag_id, dag in dagbag.dags.items() if dag_id not in paused_dag_ids
            ]
            for dag in unpaused_dags:
                dag.pickle(session)
        
        try:
            self.update_import_errors(session, dagbag)
        except Exception:  
            self.log.exception("Error logging import errors!")
        return len(dagbag.dags), len(dagbag.import_errors)
class SchedulerJob(BaseJob):  
    """
    This SchedulerJob runs for a specific time interval and schedules the jobs