**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.
airflow/models/dagrun.py
self,
        unfinished_tasks: List[TI],
        finished_tasks: List[TI],
        session: Session,
    ) -> bool:
        
        
        for ut in unfinished_tasks:
            if ut.are_dependencies_met(
                dep_context=DepContext(
                    flag_upstream_failed=True,
                    ignore_in_retry_period=True,
                    ignore_in_reschedule_period=True,
                    finished_tasks=finished_tasks),
                    session=session):
                return True
        return False
    def _emit_duration_stats_for_finished_state(self):
        if self.state == State.RUNNING:
            return
        duration = (self.end_date - self.start_date)
        if self.state is State.SUCCESS:
            Stats.timing('dagrun.duration.success.{}'.format(self.dag_id), duration)
        elif self.state == State.FAILED:
            Stats.timing('dagrun.duration.failed.{}'.format(self.dag_id), duration)
    @provide_session
    def verify_integrity(self, session: Session = None):