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
session = settings.Session()
        dag = models.DAG(dag_id='test_get_num_running_task_instances')
        dag2 = models.DAG(dag_id='test_get_num_running_task_instances_dummy')
        task = DummyOperator(task_id='task', dag=dag, start_date=DEFAULT_DATE)
        task2 = DummyOperator(task_id='task', dag=dag2, start_date=DEFAULT_DATE)
        ti1 = TI(task=task, execution_date=DEFAULT_DATE)
        ti2 = TI(task=task, execution_date=DEFAULT_DATE + datetime.timedelta(days=1))
        ti3 = TI(task=task2, execution_date=DEFAULT_DATE)
        ti1.state = State.RUNNING
        ti2.state = State.QUEUED
        ti3.state = State.RUNNING
        session.add(ti1)
        session.add(ti2)
        session.add(ti3)
        session.commit()
        self.assertEqual(1, ti1.get_num_running_task_instances(session=session))
        self.assertEqual(1, ti2.get_num_running_task_instances(session=session))
        self.assertEqual(1, ti3.get_num_running_task_instances(session=session))
    
    
    
    
    
    
    
    
    
    
    
    def test_log_url(self):