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
"""
        Get the very latest state from the database, if a session is passed,
        we use and looking up the state becomes part of the session, otherwise
        a new session is used.
        :param session: SQLAlchemy ORM Session
        :type session: Session
        """
        ti = (
            session.query(TaskInstance)
            .filter(
                TaskInstance.dag_id == self.dag_id,
                TaskInstance.task_id == self.task_id,
                TaskInstance.execution_date == self.execution_date,
            )
            .all()
        )
        if ti:
            state = ti[0].state
        else:
            state = None
        return state
    @provide_session
    def error(self, session=None):