feat: parallelism in batches

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• replaced cooperative concurrency impl with real parallelism via tokios JoinSet
• added some tests

Reference issue to close (if applicable)

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Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
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