Dealing with `get_featured_comment_posts` cache issues:

- Simplified arguments for `get_featured_comment_posts` since we weren't using the additional ones anyway
	- post_id is now the only argument
- Cache key for `get_featured_comment_posts` is post_id
- Cache is cleared on unfeature/feature comment for a given parent post_id

https://github.com/GigaOM/gigaom/issues/5097
https://github.com/GigaOM/gigaom/issues/5104
This feels like it deserves a convenience method, something like `delete_featured_comment_posts_cache( $post_id )`. Thoughts?