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
{'A': State.SUCCESS, 'B': State.NONE, 'C': State.NONE, 'D': State.NONE},
                None,
                "D -> C -> B & A -> B, when A runs but C isn't QUEUED yet, B shouldn't be QUEUED.",
            ),
            (
                {('scheduler', 'schedule_after_task_execution'): 'True'},
                {'A': 'C', 'B': 'C'},
                {'A': State.QUEUED, 'B': State.FAILED, 'C': State.NONE},
                {'A': State.SUCCESS, 'B': State.FAILED, 'C': State.UPSTREAM_FAILED},
                None,
                "A -> C & B -> C, when A is QUEUED but B has FAILED, C is marked UPSTREAM_FAILED.",
            ),
        ]
    )
    def test_fast_follow(
        self, conf, dependencies, init_state, first_run_state, second_run_state, error_message
    ):
        with conf_vars(conf):
            session = settings.Session()
            dag = DAG('test_dagrun_fast_follow', start_date=DEFAULT_DATE)
            dag_model = DagModel(
                dag_id=dag.dag_id,
                next_dagrun=dag.start_date,
                is_active=True,
            )
            session.add(dag_model)
            session.flush()
            python_callable = lambda: True
            with dag:
                task_a = PythonOperator(task_id='A', python_callable=python_callable)