**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 that dagbag.sync_to_db is retried on OperationalError"""
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        mock_dagbag.return_value.dags = {'example_dag': mock.ANY}
        op_error = OperationalError(statement=mock.ANY, params=mock.ANY, orig=mock.ANY)
        
        side_effect = [op_error, op_error, mock.ANY]
        mock_sync_to_db = mock.Mock(side_effect=side_effect)
        mock_dagbag.return_value.sync_to_db = mock_sync_to_db
        dag_file_processor.process_file("/dev/null", callback_requests=mock.MagicMock())
        mock_sync_to_db.assert_has_calls([mock.call(), mock.call(), mock.call()])
    def test_should_mark_dummy_task_as_success(self):
        dag_file = os.path.join(
            os.path.dirname(os.path.realpath(__file__)), '../dags/test_only_dummy_tasks.py'
        )
        
        with mock.patch("airflow.models.dagbag.settings.STORE_SERIALIZED_DAGS", return_value=True):