Worker threads for run_check_published_api_changes

[rudolf]

Summary

Runs each plugin's api-extractor/api-documenter in a separate worker thread. Reduces the time to check and generate docs from 3m04s to 1m50s. babel-register uses mkdir which is incompatible with Node.js 10 (nodejs/node#25526) so this is blocked on #61587

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser
• This was checked for cross-browser compatibility, including a check against IE11

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[kibanamachine]

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• Commit: f6df224
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