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


airflow/models/taskinstance.py
) -> None:
        """
        Make an XCom available for tasks to pull.
        :param key: A key for the XCom
        :type key: str
        :param value: A value for the XCom. The value is pickled and stored
            in the database.
        :type value: any picklable object
        :param execution_date: if provided, the XCom will not be visible until
            this date. This can be used, for example, to send a message to a
            task on a future date without it being immediately visible.
        :type execution_date: datetime
        :param session: Sqlalchemy ORM Session
        :type session: Session
        """
        if execution_date and execution_date < self.execution_date:
            raise ValueError(
                'execution_date can not be in the past (current '
                'execution_date is {}; received {})'.format(self.execution_date, execution_date)
            )
        XCom.set(
            key=key,
            value=value,
            task_id=self.task_id,
            dag_id=self.dag_id,
            execution_date=execution_date or self.execution_date,
            session=session,
        )
    @provide_session
    def xcom_pull(