openlineage: use `ProcessPoolExecutor` over `ThreadPoolExecutor`.

[JDarDagran]

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This may possibly fix #39232. I tested the solution many times locally and deployed in Astro Cloud (so it gives me quite certainty it works).

I don't really know how to write tests for the change (load test is the thing but do we do this in regular tests?). Some guidance would be very much helpful.