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
done, fail = False, False
        run_ti_and_assert(date1, date1, date1, 0, State.UP_FOR_RESCHEDULE, 0, 1)
        done, fail = False, False
        run_ti_and_assert(date2, date1, date2, 60, State.UP_FOR_RESCHEDULE, 0, 2)
        done, fail = False, False
        run_ti_and_assert(date3, date1, date3, 120, State.UP_FOR_RESCHEDULE, 0, 3)
        done, fail = True, False
        run_ti_and_assert(date4, date1, date4, 180, State.SUCCESS, 1, 0)
        
        dag.clear()
        ti.refresh_from_db()
        self.assertEqual(ti.state, State.NONE)
        self.assertEqual(ti._try_number, 1)
        
        
        
        done, fail = False, False
        run_ti_and_assert(date1, date1, date1, 0, State.UP_FOR_RESCHEDULE, 1, 1)
        done, fail = False, True
        run_ti_and_assert(date2, date1, date2, 60, State.UP_FOR_RETRY, 2, 0)
        done, fail = False, False
        run_ti_and_assert(date3, date3, date3, 0, State.UP_FOR_RESCHEDULE, 2, 1)
        done, fail = True, False
        run_ti_and_assert(date4, date3, date4, 60, State.SUCCESS, 3, 0)
    def test_reschedule_handling_clear_reschedules(self):