Flag custom related posts functions on VIP

[nickdaugherty]

Should be using VIP Related Posts rather than a custom implementation, which is slower and not as accurate.

A good enough implementation is to check all function names for related_posts variants, including relatedposts.

For more accurate detection, should check the body of that function for custom queries, and make sure it's not a wrapper for VIP Related Posts by checking for a WPCOM_Related_Posts()->get_related_posts() call.