**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
dagbag = DagBag(dag_folder=dag_file, include_examples=False)
            dagbag.sync_to_db()
        scheduler_job = SchedulerJob()
        scheduler_job.processor_agent = mock.MagicMock()
        dag = scheduler_job.dagbag.get_dag("test_only_dummy_tasks")
        
        session = settings.Session()
        orm_dag = session.query(DagModel).get(dag.dag_id)
        scheduler_job._create_dag_runs([orm_dag], session)
        drs = DagRun.find(dag_id=dag.dag_id, session=session)
        assert len(drs) == 1
        dr = drs[0]
        
        scheduler_job._schedule_dag_run(dr, 0, session)
        with create_session() as session:
            tis = session.query(TaskInstance).all()
        dags = scheduler_job.dagbag.dags.values()
        self.assertEqual(['test_only_dummy_tasks'], [dag.dag_id for dag in dags])
        self.assertEqual(5, len(tis))
        self.assertEqual({
            ('test_task_a', 'success'),
            ('test_task_b', None),
            ('test_task_c', 'success'),
            ('test_task_on_execute', 'scheduled'),
            ('test_task_on_success', 'scheduled'),
        }, {(ti.task_id, ti.state) for ti in tis})
        for state, start_date, end_date, duration in [(ti.state, ti.start_date, ti.end_date, ti.duration) for
                                                      ti in tis]:
            if state == 'success':
                self.assertIsNotNone(start_date)