Concurrent processing assets [3.x backport]

[wjordan]

This PR enables concurrent processing of assets, backported to target the latest stable (3.x) release of Sprockets. Similar in scope to #469 (which targets the master branch).

Use-case is the same as #469, to allow time-consuming custom Processor steps (in my use-case, an image-optimization preprocessor provided by image_optim_rails) to leverage multiple CPU cores for performance gains.

Notes:

• Since the concurrent Exporter/Writer code in the 3.x branch is not controlled by any configuration option, the concurrency in this PR is similarly non-configurable. Open to ideas/suggestions on whether/how to improve this.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @schneems (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[rails-bot]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against 3.x. Please double check that you specified the right target!

[schneems]

I'm hesitant to backport this feature as there have been some bug reports with it on sprockets 4. Let's keep new feature development to Sprockets 4. Thanks for the PR, sorry for the extreme delay.