Optimizing memory usage

[davefx]

Removing the potential undesired retrieval of hundreds or thousands of unreadable WC_Product objects into memory just to filter them out immediately.
This change prevented some out-of-memory situations in our site.

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• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' 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:

Avoid bringing into memory the whole list of WC_Products for all existing products in the database before filtering it. And instead of having the whole list in memory, better bring the products one by one.

Closes #28176 .

How to test the changes in this Pull Request:

1. Things should keep working when using /wp-admin/wp-ajax.php?action=woocommerce_json_search_products, but the amount of memory used should be dramatically reduced

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 run tests with your changes locally?

Changelog entry

• Tweak - Reduce memory usage when executing json_search_products and json_search_products_and_variations

[roykho]

This looks good. Thanks for the contribution. With that I just want to note down that while this fix indeed improves performance in terms of memory usage, it can still run out of memory if the search is large enough.

But here is a back to back test I did comparing master against this PR running against 3000 products.

master
[09-Nov-2020 22:41:19 UTC] memory usage: 192104832
[09-Nov-2020 22:43:16 UTC] memory usage: 192126632
[09-Nov-2020 22:44:10 UTC] memory usage: 192104928
[09-Nov-2020 22:45:39 UTC] memory usage: 192132896

This PR
[09-Nov-2020 22:47:38 UTC] memory usage: 153366632
[09-Nov-2020 22:48:08 UTC] memory usage: 153366632
[09-Nov-2020 22:48:30 UTC] memory usage: 153366632
[09-Nov-2020 22:48:58 UTC] memory usage: 153366632