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 = DAG(
            'test_task_start_end_stats',
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=10),
        )
        op = DummyOperator(task_id='dummy_op', dag=dag)
        dag.create_dagrun(
            run_id='manual__' + DEFAULT_DATE.isoformat(),
            execution_date=DEFAULT_DATE,
            start_date=DEFAULT_DATE,
            state=State.RUNNING,
            external_trigger=False,
        )
        ti = TI(task=op, execution_date=DEFAULT_DATE)
        ti.state = State.RUNNING
        session = settings.Session()
        session.merge(ti)
        session.commit()
        ti._run_raw_task()
        ti.refresh_from_db()
        stats_mock.assert_called_with(f'ti.finish.{dag.dag_id}.{op.task_id}.{ti.state}')
        self.assertIn(call(f'ti.start.{dag.dag_id}.{op.task_id}'), stats_mock.mock_calls)
        self.assertEqual(stats_mock.call_count, 5)
    def test_generate_command_default_param(self):