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
called = False
        def on_execute_callable(context):
            nonlocal called
            called = True
            self.assertEqual(context['dag_run'].dag_id, 'test_dagrun_execute_callback')
        dag = DAG(
            'test_execute_callback',
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=10),
        )
        task = DummyOperator(
            task_id='op', email='test@test.test', on_execute_callback=on_execute_callable, dag=dag
        )
        ti = TI(task=task, execution_date=datetime.datetime.now())
        ti.state = State.RUNNING
        session = settings.Session()
        dag.create_dagrun(
            execution_date=ti.execution_date,
            state=State.RUNNING,
            run_type=DagRunType.SCHEDULED,
            session=session,
        )
        session.merge(ti)
        session.commit()
        ti._run_raw_task()
        assert called
        ti.refresh_from_db()
        assert ti.state == State.SUCCESS
    def test_handle_failure(self):