**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
assert isinstance(orm_dag.next_dagrun, datetime.datetime)
        
        dr.start_date = timezone.utcnow() - datetime.timedelta(days=1)
        session.flush()
        
        scheduler.processor_agent = mock.Mock()
        scheduler.processor_agent.send_callback_to_execute = mock.Mock()
        scheduler._schedule_dag_run(dr, 0, session)
        session.flush()
        session.refresh(dr)
        assert dr.state == State.FAILED
        session.refresh(orm_dag)
        assert isinstance(orm_dag.next_dagrun, datetime.datetime)
        assert isinstance(orm_dag.next_dagrun_create_after, datetime.datetime)
        expected_callback = DagCallbackRequest(
            full_filepath=dr.dag.fileloc,
            dag_id=dr.dag_id,
            is_failure_callback=True,
            execution_date=dr.execution_date,
            msg="timed_out"
        )
        
        scheduler.processor_agent.send_callback_to_execute.assert_called_once_with(expected_callback)
        session.rollback()
        session.close()
    def test_dagrun_timeout_fails_run(self):
        """
        Test if a a dagrun will be set failed if timeout, even without max_active_runs
        """