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._kill_process()
        if not self._process.is_alive():
            self._done = True
            self.log.debug("Waiting for %s", self._process)
            self._process.join()
            self._parent_channel.close()
            return True
        return False
    @property
    def result(self) -> Optional[Tuple[int, int]]:
        """
        :return: result of running SchedulerJob.process_file()
        :rtype: tuple[int, int] or None
        """
        if not self.done:
            raise AirflowException("Tried to get the result before it's done!")
        return self._result
    @property
    def start_time(self) -> datetime.datetime:
        """
        :return: when this started to process the file
        :rtype: datetime
        """
        if self._start_time is None:
            raise AirflowException("Tried to get start time before it started!")
        return self._start_time
    @property
    def waitable_handle(self):
        return self._process.sentinel
class DagFileProcessor(LoggingMixin):