Added MySQL index hint to use ti_state on find_zombies query

[michaelmicheal]

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Similar to this issue and the associated PR, we are seeing MySQL is not using the ti_state index and completing a full-table scan on the task_instance table on the find_zombies query when the ti_state index should be used to find tasks in running state.

zombies = (
            session.query(TaskInstance, DagModel.fileloc)
            .join(LocalTaskJob, TaskInstance.job_id == LocalTaskJob.id)
            .join(DagModel, TaskInstance.dag_id == DagModel.dag_id)
            .filter(TaskInstance.state == TaskInstanceState.RUNNING)
            .filter(
                or_(
                    LocalTaskJob.state != State.RUNNING,
                    LocalTaskJob.latest_heartbeat < limit_dttm,
                )
            )
            .filter(TaskInstance.queued_by_job_id == self.id)
            .all()
        )

Adding a MySQL index hint resolved this issue for us, so this PR adds an index hint on the above query.