**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
"""Test SLA Callbacks are not sent when no task SLAs are defined"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_no_task_slas'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag)
        with patch.object(settings, "CHECK_SLAS", True):
            scheduler_job = SchedulerJob()
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_not_called()
    def test_send_sla_callbacks_to_processor_sla_with_task_slas(self):
        """Test SLA Callbacks are sent to the DAG Processor when SLAs are defined on tasks"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_with_task_slas'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag, sla=timedelta(seconds=60))
        with patch.object(settings, "CHECK_SLAS", True):
            scheduler_job = SchedulerJob()
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_called_once_with(
                full_filepath=dag.fileloc, dag_id=dag_id
            )
    def test_scheduler_sets_job_id_on_dag_run(self):