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


tests/models/test_taskinstance.py
dag = models.DAG(dag_id='test_failure_email')
        task = BashOperator(
            task_id='test_email_alert_with_config',
            dag=dag,
            bash_command='exit 1',
            start_date=DEFAULT_DATE,
            email='to',
        )
        ti = TI(task=task, execution_date=timezone.utcnow())
        opener = mock_open(read_data='template: {{ti.task_id}}')
        with patch('airflow.models.taskinstance.open', opener, create=True):
            try:
                ti.run()
            except AirflowException:
                pass
        (email, title, body), _ = mock_send_email.call_args
        self.assertEqual(email, 'to')
        self.assertEqual('template: test_email_alert_with_config', title)
        self.assertEqual('template: test_email_alert_with_config', body)
    def test_set_duration(self):