Fix course structure ordering cache

[renatho]

Fixes #3891

Changes proposed in this Pull Request

• It fixes the issue related to cache when ordering the course structure.
  • Notice that it only happens when using memcached for object cache. With Redis, using the plugin "Redis Object Cache" I couldn't reproduce the issue.
• The chosen strategy was to avoid cache in some specific cases while getting the structure for edit. It was done by adding a new conditional to the WHERE query. It seems the way that solves for any extension. My first option was trying to clear some cache objects, but in the plugin I used, for example, it had some specific names, so we couldn't clear it in a generic way for any object cache extension.

Testing instructions

You can configure your local env (2 first steps), or you can test it in Pressable or an Ephemeral site.

• Configure and start memcached.
• Configure the object cache in WordPress to use the memcached. I used the plugin Docket Cache for this.
• Create a course with modules and lessons (use lessons inside modules and lessons without modules to make sure all scenarios are working properly).
• Reorder the lessons, save the post, and make sure the lessons continue in the correct place (see the issue for more details).
• Go to WP admin > Lessons > Order Lessons.
• Select your course.
• Reorder the lessons, and make sure it works properly.

[jom]

This works well! I had one tiny comment, but not a big deal at all if you think you'd prefer merging as-is.

[jom]

I don't think this would ever happen, but I wonder if it is a tiny-tiny-tiny bit safer to set time() to a variable here. I mean, we'd have to be really unlucky and I doubt PHP would ever do that.

[renatho]

Ah, of course! Fixed here: c0fffe1

[jom]

🚢