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)
        dag2 = DAG('dag2', start_date=DEFAULT_DATE)
        op1 = DummyOperator(task_id='test_op_1', owner='test')
        op2 = DummyOperator(task_id='test_op_2', owner='test')
        op3 = DummyOperator(task_id='test_op_3', owner='test', dag=dag)
        op4 = DummyOperator(task_id='test_op_4', owner='test', dag=dag2)
        
        self.assertEqual([i.has_dag() for i in [op1, op2, op3, op4]], [False, False, True, True])
        
        self.assertRaises(AirflowException, op1.set_downstream, op2)
        
        op1.dag = dag
        op1.set_downstream(op2)
        self.assertIs(op2.dag, dag)
        
        self.assertRaises(AirflowException, op1.set_downstream, op4)
        self.assertRaises(AirflowException, op1.set_downstream, [op3, op4])
    def test_bitshift_compose_operators(self):