[AIRFLOW-6874] Reap cgroup procs when terminate in cgroup taskrunner

[YingboWang]

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Issue link: AIRFLOW-6874

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Many airflow tasks create subprocesses and these subprocesses may create more subprocesses. In our experience, there is a risk that although a task failed and tried to reap the process group, there are still left over processes running and cause issues with both resources and correctness.

Propose to improve the cgroup task runner to reap all processes for current node on node termination.

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• Unit tests coverage for changes (not needed for documentation changes)
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^* For document-only changes commit message can start with [AIRFLOW-XXXX].

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[KevinYang21]

This is very valuable change. Can we have some unit test around this change and have a green light on the CI?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.