Add defensive copy for Futures allOf() method

[jinkshower]

Closes #2935

I've added defensive copy for the method so that external modification could not affect stages during iteration.

As for test, I can't come up with any better idea so just used two Threads each calling allOf() and modifying stages simultaneously.

Thank you.

Make sure that:

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• You have created a feature request first to discuss your contribution intent. Please reference the feature request ticket number in the pull request.
• You applied code formatting rules using the mvn formatter:format target. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.

[jinkshower]

I've applied provided formatter again! Could you re-run the test please? Thank you in advance.

[tishun]

I've applied provided formatter again! Could you re-run the test please? Thank you in advance.

You need to fetch the fix from #2949

[jinkshower]

You need to fetch the fix from #2949

I've fetched #2949 and pushed it again!

[tishun]

Thanks for the contribution!

[jinkshower]

neat test! Thanks for the polishing commit, learned a lot