Promote object caching in Site Health

[tillkruss]

Recommend the use of an object cache under Site Health for sites with non-trivial amounts of data.

• Detect available extensions and make recommendations
  • See AMP
• What are the criteria for running the check?
• Allow WooCommerce (etc.) to enable running the check?

Would you see this as being something that checks for APCu, or a reachable memcache / redis install?

[tillkruss]

Per @westonruter:

There is heavy transient usage in the AMP plugin. Whenever there is an img that lacks a width and height attribute, the plugin will fetch the image to obtain the dimensions and store them in a transient. Also, during stylesheet processing, it will use php-css-parser to parse the stylesheets and since the parsing is computationally-expensive, it stores the result in transients as well. So without persistent object caching, the wp_options table can get huge indeed.

[eclarke1]

@tillkruss if it's ok I just removed the 'Needs Discussion' label here as it looks like it's been started with a draft PR