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
num_queued_tis = self._do_scheduling(session)
                self.executor.heartbeat()
                session.expunge_all()
                num_finished_events = self._process_executor_events(session=session)
            self.processor_agent.heartbeat()
            
            self.heartbeat(only_if_necessary=True)
            self._emit_pool_metrics()
            loop_end_time = time.time()
            loop_duration = loop_end_time - loop_start_time
            self.log.debug("Ran scheduling loop in %.2f seconds", loop_duration)
            if not is_unit_test and not num_queued_tis and not num_finished_events:
                
                
                
                time.sleep(self._processor_poll_interval)
            if loop_count >= self.num_runs > 0:
                self.log.info(
                    "Exiting scheduler loop as requested number of runs (%d - got to %d) has been reached",
                    self.num_runs, loop_count,
                )
                break
            if self.processor_agent.done:
                self.log.info(
                    "Exiting scheduler loop as requested DAG parse count (%d) has been reached after %d "
                    " scheduler loops",
                    self.num_times_parse_dags, loop_count,
                )
                break
    def _do_scheduling(self, session) -> int: