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.
tests/jobs/test_scheduler_job.py
"""Test SLA Callbacks are not sent when no task SLAs are defined"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_no_task_slas'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag)
        with patch.object(settings, "CHECK_SLAS", True):
            scheduler_job = SchedulerJob()
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_not_called()
    def test_send_sla_callbacks_to_processor_sla_with_task_slas(self):
        """Test SLA Callbacks are sent to the DAG Processor when SLAs are defined on tasks"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_with_task_slas'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag, sla=timedelta(seconds=60))
        with patch.object(settings, "CHECK_SLAS", True):
            scheduler_job = SchedulerJob()
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_called_once_with(
                full_filepath=dag.fileloc, dag_id=dag_id
            )
    def test_scheduler_sets_job_id_on_dag_run(self):