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
for (dep_patch, method_patch) in patch_dict.values():
            dep_patch.stop()
    def test_mark_non_runnable_task_as_success(self):
        """
        test that running task with mark_success param update task state
        as SUCCESS without running task despite it fails dependency checks.
        """
        non_runnable_state = (set(State.task_states) - RUNNABLE_STATES - set(State.SUCCESS)).pop()
        dag = models.DAG(dag_id='test_mark_non_runnable_task_as_success')
        task = DummyOperator(
            task_id='test_mark_non_runnable_task_as_success_op',
            dag=dag,
            pool='test_pool',
            owner='airflow',
            start_date=timezone.datetime(2016, 2, 1, 0, 0, 0),
        )
        ti = TI(task=task, execution_date=timezone.utcnow(), state=non_runnable_state)
        
        with create_session() as session:
            session.add(ti)
            dag.create_dagrun(
                execution_date=ti.execution_date,
                state=State.RUNNING,
                run_type=DagRunType.SCHEDULED,
                session=session,
            )
            session.commit()
        ti.run(mark_success=True)
        self.assertEqual(ti.state, State.SUCCESS)
    def test_run_pooling_task(self):