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
self.unixname = ti.unixname
            self.job_id = ti.job_id
            self.pool = ti.pool
            self.pool_slots = ti.pool_slots or 1
            self.queue = ti.queue
            self.priority_weight = ti.priority_weight
            self.operator = ti.operator
            self.queued_dttm = ti.queued_dttm
            self.pid = ti.pid
        else:
            self.state = None
        self.log.debug("Refreshed TaskInstance %s", self)
    def refresh_from_task(self, task, pool_override=None):
        """
        Copy common attributes from the given task.
        :param task: The task object to copy from
        :type task: airflow.models.BaseOperator
        :param pool_override: Use the pool_override instead of task's pool
        :type pool_override: str
        """
        self.queue = task.queue
        self.pool = pool_override or task.pool
        self.pool_slots = task.pool_slots
        self.priority_weight = task.priority_weight_total
        self.run_as_user = task.run_as_user
        self.max_tries = task.retries
        self.executor_config = task.executor_config
        self.operator = task.task_type
    @provide_session
    def clear_xcom_data(self, session=None):