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
retry_delay=delay,
            retry_exponential_backoff=True,
            max_retry_delay=max_delay,
            dag=dag,
            owner='airflow',
            start_date=timezone.datetime(2016, 2, 1, 0, 0, 0),
        )
        ti = TI(task=task, execution_date=DEFAULT_DATE)
        ti.end_date = pendulum.instance(timezone.utcnow())
        date = ti.next_retry_datetime()
        
        period = ti.end_date.add(seconds=30) - ti.end_date.add(seconds=15)
        self.assertTrue(date in period)
        ti.try_number = 3
        date = ti.next_retry_datetime()
        
        period = ti.end_date.add(seconds=240) - ti.end_date.add(seconds=120)
        self.assertTrue(date in period)
        ti.try_number = 5
        date = ti.next_retry_datetime()
        
        period = ti.end_date.add(seconds=960) - ti.end_date.add(seconds=480)
        self.assertTrue(date in period)
        ti.try_number = 9
        date = ti.next_retry_datetime()
        self.assertEqual(date, ti.end_date + max_delay)
        ti.try_number = 50
        date = ti.next_retry_datetime()
        self.assertEqual(date, ti.end_date + max_delay)
    def test_next_retry_datetime_short_intervals(self):