**Apache Airflow version**: 2.0.0a1

**What happened**:

On my dev environment I have a lot of DAG runs with failed tasks, where the *downstream* task instances then have been set to `upstream_failed`. The scheduler is now refusing to schedule more instances for such DAGs.

The scheduler looks for such task instances that are not in the `State.finished` set:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/jobs/scheduler_job.py#L1482-L1488

However, `State.finished` is currently not a proper inverse of *running* states, it only includes success, failed, and skipped:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/utils/state.py#L110-L114

What is missing is the task instance `upstream_failed` state, so currently my log is being filled (very rapidly) with:

```
[2020-10-18 13:22:18,503] {{scheduler_job.py:1658}} INFO - DAG test_dag already has 26 active runs, not queuing any more tasks
```

Those 26 'active runs' are simply dag runs that failed and have 'upstream_failed' task instances.

Either `upstream_failed` should be added to `State.finished`, or the Scheduler task instance test should explicitly take that state into account.
tests/jobs/test_scheduler_job.py
bf_exec = MockExecutor()
            backfill = BackfillJob(
                executor=bf_exec,
                dag=dag,
                start_date=DEFAULT_DATE,
                end_date=DEFAULT_DATE)
            backfill.run()
            
            self.assertEqual(
                len(session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id).all()), 1)
            self.assertListEqual(
                [
                    (TaskInstanceKey(dag.dag_id, 'dummy', DEFAULT_DATE, 1), (State.SUCCESS, None)),
                ],
                bf_exec.sorted_tasks
            )
            session.commit()
            scheduler = SchedulerJob(dag.fileloc,
                                     executor=self.null_exec,
                                     num_runs=1)
            scheduler.run()
            
            self.assertEqual(
                len(session.query(TaskInstance).filter(TaskInstance.dag_id == dag_id).all()), 1)
            session.commit()
            self.assertListEqual([], self.null_exec.sorted_tasks)
    def test_scheduler_task_start_date(self):