Search: Fix race condition for API responses

[jsnmoon]

Changes proposed in this Pull Request:

Currently, it's possible for stale API responses to overwrite newer API responses. As a fix, this PR makes two changes:

1. Attaches a client-generated sequential ID to each API response to determine whether or not to evict the existing response stored in the Redux store.
2. Prevents the getResults denounce from being flushed upon SearchApp mounting.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Apply this change to your site with a Jetpack Search subscription.
• Directly navigate to a search page (e.g. ?s=apples).
• Ensure that only one search API response is recorded in the browser network console.
• Ensure that an older API response never overwrites a newer API response; this might be hard to reproduce. To make this easier, you can apply this patch to add some randomized delay to each API response.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jsnmoon! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D59942-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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Jetpack plugin:

• Next scheduled release: April 13, 2021.
• Scheduled code freeze: April 6, 2021.

[bluefuton]

👍 Tests out well.

[jeherve]

This tests well for me. 🚢

[github-actions]

Great news! One last step: head over to your WordPress.com diff, D59942-code, and commit it.
Once you've done so, come back to this PR and add a comment with your changeset ID.

Thank you!

[jsnmoon]

🚢 r224082-wpcom