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
"""
        test that try to create a task with pool_slots less than 1
        """
        def create_task_instance():
            dag = models.DAG(dag_id='test_run_pooling_task')
            task = DummyOperator(
                task_id='test_run_pooling_task_op',
                dag=dag,
                pool='test_pool',
                pool_slots=0,
                owner='airflow',
                start_date=timezone.datetime(2016, 2, 1, 0, 0, 0),
            )
            return TI(task=task, execution_date=timezone.utcnow())
        self.assertRaises(AirflowException, create_task_instance)
    @provide_session
    def test_ti_updates_with_task(self, session=None):
        """
        test that updating the executor_config propogates to the TaskInstance DB
        """
        with models.DAG(dag_id='test_run_pooling_task') as dag:
            task = DummyOperator(
                task_id='test_run_pooling_task_op',
                owner='airflow',
                executor_config={'foo': 'bar'},
                start_date=timezone.datetime(2016, 2, 1, 0, 0, 0),