**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/ti_deps/dep_context.py
:type ignore_in_reschedule_period: bool
    :param ignore_task_deps: Ignore task-specific dependencies such as depends_on_past and
        trigger rule
    :type ignore_task_deps: bool
    :param ignore_ti_state: Ignore the task instance's previous failure/success
    :type ignore_ti_state: bool
    :param finished_tasks: A list of all the finished tasks of this run
    :type finished_tasks: list[airflow.models.TaskInstance]
    """
    def __init__(
            self,
            deps=None,
            flag_upstream_failed: bool = False,
            ignore_all_deps: bool = False,
            ignore_depends_on_past: bool = False,
            ignore_in_retry_period: bool = False,
            ignore_in_reschedule_period: bool = False,
            ignore_task_deps: bool = False,
            ignore_ti_state: bool = False,
            finished_tasks=None):
        self.deps = deps or set()
        self.flag_upstream_failed = flag_upstream_failed
        self.ignore_all_deps = ignore_all_deps
        self.ignore_depends_on_past = ignore_depends_on_past
        self.ignore_in_retry_period = ignore_in_retry_period
        self.ignore_in_reschedule_period = ignore_in_reschedule_period
        self.ignore_task_deps = ignore_task_deps
        self.ignore_ti_state = ignore_ti_state
        self.finished_tasks = finished_tasks
    def ensure_finished_tasks(self, dag, execution_date: pendulum.DateTime, session: Session):