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
"""
        Forces the task instance's state to FAILED in the database.
        :param session: SQLAlchemy ORM Session
        :type session: Session
        """
        self.log.error("Recording the task instance as FAILED")
        self.state = State.FAILED
        session.merge(self)
        session.commit()
    @provide_session
    def refresh_from_db(self, session=None, lock_for_update=False) -> None: