Pramen pipelines sometimes hang when parallel.tasks > 1

## 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

```hocon
pramen {
parallel.tasks = 4
}
```

## Expected behavior
Pramen should never hang, at least internally.

## Context
To be added...