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
self,
        task_instances: List[TI]
    ) -> None:
        """
        Takes task_instances, which should have been set to queued, and enqueues them
        with the executor.
        :param task_instances: TaskInstances to enqueue
        :type task_instances: list[TaskInstance]
        """
        
        for ti in task_instances:
            command = TI.generate_command(
                ti.dag_id,
                ti.task_id,
                ti.execution_date,
                local=True,
                mark_success=False,
                ignore_all_deps=False,
                ignore_depends_on_past=False,
                ignore_task_deps=False,
                ignore_ti_state=False,
                pool=ti.pool,
                file_path=ti.dag_model.fileloc,
                pickle_id=ti.dag_model.pickle_id,
            )
            priority = ti.priority_weight
            queue = ti.queue
            self.log.info(
                "Sending %s to executor with priority %s and queue %s",
                ti.key, priority, queue