**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
dag2.sync_to_db(session=session)
        ti1 = TaskInstance(task1, DEFAULT_DATE)
        ti1.state = State.QUEUED
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.FAILED, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        self.assertEqual(ti1.state, State.QUEUED)
        mock_task_callback.assert_called_once_with(
            full_filepath='/test_path1/',
            simple_task_instance=mock.ANY,
            msg='Executor reports task instance '
                '<TaskInstance: test_process_executor_events.dummy_task 2016-01-01 00:00:00+00:00 [queued]> '
                'finished (failed) although the task says its queued. (Info: None) '
                'Was the task killed externally?'
        )
        scheduler.processor_agent.send_callback_to_execute.assert_called_once_with(task_callback)
        scheduler.processor_agent.reset_mock()
        
        ti1.state = State.SUCCESS
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.SUCCESS, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        self.assertEqual(ti1.state, State.SUCCESS)
        scheduler.processor_agent.send_callback_to_execute.assert_not_called()
        mock_stats_incr.assert_called_once_with('scheduler.tasks.killed_externally')
    def test_process_executor_events_uses_inmemory_try_number(self):