Add Site Health check module for persistent object cache

[tillkruss]

Summary

This PR adds the object-cache/health module containing a single health check to suggest the usage of a persistent object cache backend, if needed.

The module adds the direct persistent_object_cache health check.

• Plugins (like WooCommerce) can use the site_status_suggest_persistent_object_cache filter to inject their own criteria checks to suggest the use of an object cache, as well as bypass the default core health check
• Hosts may use the site_status_persistent_object_cache_url filter to link to their own support docs
• Hosts may want to use the site_status_persistent_object_cache_notes filter to recommend their preferred object caching solution
• Plugin authors may want to append notes using the site_status_persistent_object_cache_notes filter on why object caching is recommended for their plugin

Resolves #35.

[spacedmonkey]

@tillkruss Can you explain what I am meant to see in site health. I checked two sites and I see any new options in site health?

Can you provide details to test this and maybe a screenshot?

[felixarntz]

@tillkruss LGTM, just a few last nit-picks.

[tillkruss]

@felixarntz: All done. Any filter you'd like me to remove?

[spacedmonkey]

Good first PR.

[felixarntz]

@tillkruss I think all filters currently present make sense, except for the one I'm flagging below. Particularly the filters to modify the Site Health contents are great to have.

If you feel strongly about keeping this filter, I'd say at a minimum we should put that into the perflab_oc_health_should_persistent_object_cache() function as a "short-circuit" filter, rather than hooking that function onto the filter.

[felixarntz]

@tillkruss Production code lgtm, leaving a few recommendations on tests. At a minimum, we should fix the one filter usage there that's using the wrong filter.

[tillkruss]

@tillkruss Production code lgtm, leaving a few recommendations on tests. At a minimum, we should fix the one filter usage there that's using the wrong filter.

@felixarntz: All resolved in 6291513.

[felixarntz]

@tillkruss Awesome stuff! 🎉