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
self.callback_ran = True
        session = settings.Session()
        temp_instance = (
            session.query(TI)
            .filter(TI.task_id == self.task_id)
            .filter(TI.dag_id == self.dag_id)
            .filter(TI.execution_date == self.execution_date)
            .one()
        )
        self.task_state_in_callback = temp_instance.state
class TestTaskInstance(unittest.TestCase):
    @staticmethod
    def clean_db():
        db.clear_db_dags()
        db.clear_db_pools()
        db.clear_db_runs()
        db.clear_db_task_fail()
        db.clear_rendered_ti_fields()
        db.clear_db_task_reschedule()
    def setUp(self):
        self.clean_db()
        with create_session() as session:
            test_pool = Pool(pool='test_pool', slots=1)
            session.add(test_pool)
            session.commit()
    def tearDown(self):
        self.clean_db()
    def test_set_task_dates(self):