**Apache Airflow version**: 2.0.0a2

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): v1.15.12

**Environment**:
- **OS** (e.g. from /etc/os-release): CentOS 7


**What happened**:

When using KubernetesExecutor, a single SIGTERM doesn't result in the scheduler actually exiting cleanly. However, it will exit after receiving a second SIGTERM.

The same behavior happens if you have an exception in the scheduler. An easy way to replicate it is to raise in `pod_mutation_hook`.

In practice, this means the scheduler doesn't get restarted by K8s after encountering a exception. Or, when terminating the scheduler pod, it sticks around until k8s ultimately sends it a SIGKILL after its grace period is up.

**What you expected to happen**:

The scheduler to exit after a single SIGTERM.

**How to reproduce it**:
The easiest way to reproduce it is to simply send a SIGKILL to your KubernetesExecutor scheduler pod:

```
$ kubectl exec {scheduler_pod_name} kill 1
```

Note, the pod stays running, while the logs report:

```
[2020-10-30 17:34:09,581] {scheduler_job.py:794} INFO - Exiting gracefully upon receiving signal 15
[2020-10-30 17:34:10,586] {process_utils.py:97} INFO - Sending Signals.SIGTERM to GPID 59
[2020-10-30 17:34:10,879] {process_utils.py:63} INFO - Process psutil.Process(pid=59, status='terminated', exitcode=0, started=
'17:33:23') (59) terminated with exit code 0
[2020-10-30 17:34:10,882] {process_utils.py:97} INFO - Sending Signals.SIGTERM to GPID 59
[2020-10-30 17:34:10,882] {scheduler_job.py:1329} INFO - Exited execute loop
```

Issue another `kill`:
```
$ kubectl exec {scheduler_pod_name} kill 1
```

And the pod will be restarted by k8s.

If you can't reproduce it, let me know and I'll try and get something running on kind.

**Anything else we need to know**:

The same behavior happens if you have an exception in the scheduler. An easy way to replicate it is to raise in a `pod_mutation_hook`.


airflow/jobs/scheduler_job.py
session.query(TI)
                .filter(TI.state != State.SUCCESS, TI.execution_date.in_(sla_dates), TI.dag_id == dag.dag_id)
                .all()
            )
            blocking_tis: List[TI] = []
            for ti in fetched_tis:
                if ti.task_id in dag.task_ids:
                    ti.task = dag.get_task(ti.task_id)
                    blocking_tis.append(ti)
                else:
                    session.delete(ti)
                    session.commit()
            task_list = "\n".join([sla.task_id + ' on ' + sla.execution_date.isoformat() for sla in slas])
            blocking_task_list = "\n".join(
                [ti.task_id + ' on ' + ti.execution_date.isoformat() for ti in blocking_tis]
            )
            
            
            email_sent = False
            notification_sent = False
            if dag.sla_miss_callback:
                
                self.log.info('Calling SLA miss callback')
                try:
                    dag.sla_miss_callback(dag, task_list, blocking_task_list, slas, blocking_tis)
                    notification_sent = True
                except Exception:  
                    self.log.exception("Could not call sla_miss_callback for DAG %s", dag.dag_id)
            email_content = f"""\
            Here's a list of tasks that missed their SLAs: