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
pass
        class TestOperator(PythonOperator):
            def post_execute(self, context, result=None):
                if result == 'error':
                    raise TestError('expected error.')
        dag = models.DAG(dag_id='test_post_execute_dag')
        task = TestOperator(
            task_id='test_operator',
            dag=dag,
            python_callable=lambda: 'error',
            owner='airflow',
            start_date=timezone.datetime(2017, 2, 1),
        )
        ti = TI(task=task, execution_date=timezone.utcnow())
        with self.assertRaises(TestError):
            ti.run()
    def test_check_and_change_state_before_execution(self):