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
task = DummyOperator(task_id='op', email='test@test.test')
        ti = TI(
            task=task,
            execution_date=datetime.datetime.now(),
        )
        ti.start_date = datetime.datetime(2018, 10, 1, 1)
        ti.end_date = datetime.datetime(2018, 10, 1, 2)
        ti.set_duration()
        self.assertEqual(ti.duration, 3600)
    def test_set_duration_empty_dates(self):
        task = DummyOperator(task_id='op', email='test@test.test')
        ti = TI(task=task, execution_date=datetime.datetime.now())
        ti.set_duration()
        self.assertIsNone(ti.duration)
    def test_success_callback_no_race_condition(self):
        callback_wrapper = CallbackWrapper()
        dag = DAG(
            'test_success_callback_no_race_condition',
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=10),
        )
        task = DummyOperator(