**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
self, dagbag: DagBag, callback_requests: List[CallbackRequest], session: Session = None
    ) -> None:
        """
        Execute on failure callbacks. These objects can come from SchedulerJob or from
        DagFileProcessorManager.
        :param dagbag: Dag Bag of dags
        :param callback_requests: failure callbacks to execute
        :type callback_requests: List[airflow.utils.callback_requests.CallbackRequest]
        :param session: DB session.
        """
        for request in callback_requests:
            self.log.debug("Processing Callback Request: %s", request)
            try:
                if isinstance(request, TaskCallbackRequest):
                    self._execute_task_callbacks(dagbag, request)
                elif isinstance(request, SlaCallbackRequest):
                    self.manage_slas(dagbag.dags.get(request.dag_id))
                elif isinstance(request, DagCallbackRequest):
                    self._execute_dag_callbacks(dagbag, request, session)
            except Exception:  
                self.log.exception(
                    "Error executing %s callback for file: %s",
                    request.__class__.__name__,
                    request.full_filepath,
                )
        session.commit()
    @provide_session
    def _execute_dag_callbacks(self, dagbag: DagBag, request: DagCallbackRequest, session: Session):