Pramen pipelines sometimes hang when parallel.tasks > 1

[yruslan]

Describe the bug

There are reports that Pramen pipelines hang when parallel.tasks > 1, especially on big pipelines.

So far we couldn't reproduce it in unit tests, and don't know any more information about it.

This issue is for tracking and fixing the bug

Code and/or configuration snippet that caused the issue

pramen {
  parallel.tasks = 4
}

Expected behavior

Pramen should never hang, at least internally.

Context

To be added...