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('dag', start_date=DEFAULT_DATE)
        op1 = DummyOperator(task_id='op_1', owner='test', dag=dag)
        ti = TI(task=op1, execution_date=timezone.utcnow(), state=State.RUNNING)
        start_date = timezone.utcnow()
        ti.start_date = start_date
        ti.set_state(State.UP_FOR_RETRY)
        assert ti.state == State.UP_FOR_RETRY
        assert ti.start_date == start_date, "Start date should have been left alone"
        assert ti.start_date < ti.end_date
        assert ti.duration > 0
@pytest.mark.parametrize("pool_override", [None, "test_pool2"])
def test_refresh_from_task(pool_override):