fix(block-caching): only serve from cache if all blocks are cached

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

If only some blocks are cached, rendered blocks may be duplicated. This change ensures that the cached blocks are served only if all applicable blocks on a page can be cached.

How to test the changes in this Pull Request:

1. On release,
2. Throw the wrench in the works by returning rand( 0, 1 ) from should_cache_block method
3. Load a page with Homepage Posts blocks, observe that some are duplicated (might have to reload, depending on your luck)
4. Switch to this branch, reload, observe the blocks are never duplicated

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[claudiulodro]

Latest revision works great in my testing and code looks solid. Thanks for digging into this!

[matticbot]

🎉 This PR is included in version 4.2.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀