**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
([22,  24,  27, 30],  1,  1, "1d",   "30m",  "no_structure"),  
            ([22,  24,  27, 30],  1,  1, "1d",   "30m",        "linear"),  
            ([22,  24,  27, 30],  1,  1, "1d",   "30m",   "binary_tree"),  
            ([22,  24,  27, 30],  1,  1, "1d",   "30m",          "star"),  
            ([22,  24,  27, 30],  1,  1, "1d",   "30m",          "grid"),  
            
            ([10,  10,  10, 10],  1,  5, "1d",  "None",  "no_structure"),  
            ([10,  10,  10, 10],  1,  5, "1d",  "None",        "linear"),  
            ([22,  14,  14, 14],  1,  5, "1d", "@once",  "no_structure"),  
            ([23,  15,  15, 15],  1,  5, "1d", "@once",        "linear"),  
            ([22,  24,  27, 30],  1,  5, "1d",   "30m",  "no_structure"),  
            ([23,  26,  30, 34],  1,  5, "1d",   "30m",        "linear"),  
            ([23,  26,  30, 34],  1,  5, "1d",   "30m",   "binary_tree"),  
            ([23,  26,  30, 34],  1,  5, "1d",   "30m",          "star"),  
            ([23,  26,  30, 34],  1,  5, "1d",   "30m",          "grid"),  
            
            ([10,  10,  10,  10], 10, 10, "1d",  "None",  "no_structure"),  
            ([10,  10,  10,  10], 10, 10, "1d",  "None",        "linear"),  
            ([85,  38,  38,  38], 10, 10, "1d", "@once",  "no_structure"),  
            ([95,  51,  51,  51], 10, 10, "1d", "@once",        "linear"),  
            ([85,  99,  99,  99], 10, 10, "1d",   "30m",  "no_structure"),  
            ([95, 125, 125, 125], 10, 10, "1d",   "30m",        "linear"),  
            ([95, 119, 119, 119], 10, 10, "1d",   "30m",   "binary_tree"),  
            ([95, 119, 119, 119], 10, 10, "1d",   "30m",          "star"),  
            ([95, 119, 119, 119], 10, 10, "1d",   "30m",          "grid"),  
            
        ]
    )
    @pytest.mark.quarantined
    def test_process_dags_queries_count(