For xcoms pushed from different tasks, having the same execution_date and in the same Dag, the result of the xcom_pull with key set as None is not deterministic in its order

**Apache Airflow version**: 2.0


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

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
When tasks in the same dag push Xcoms, the result of the xcom_pull comes in any order. The order is not deterministic.

**What you expected to happen**:

I expected the order to be deterministic.

**How to reproduce it**:

Run the provided example_xcom dag in the repository multiple times


airflow/models/taskinstance.py
if test_mode:
            return
        self.end_date = timezone.utcnow()
        self.set_duration()
        
        session.add(
            TaskReschedule(
                self.task,
                self.execution_date,
                self._try_number,
                actual_start_date,
                self.end_date,
                reschedule_exception.reschedule_date,
            )
        )
        
        self.state = State.UP_FOR_RESCHEDULE
        
        
        self._try_number -= 1
        session.merge(self)
        session.commit()
        self.log.info('Rescheduling task, marking task as UP_FOR_RESCHEDULE')
    @provide_session
    def handle_failure(self, error, test_mode=None, context=None, force_fail=False, session=None):