Prime caches on post save

[stevegrunwell]

Rather than relying on the first visit to a taxonomy archive page to populate the cache, we can prime it automatically at the end of Meta\save_post(): instead of clearing the caches for any term ID that's been affected, we can call Display\get_sticky_posts_for_term() to populate the cache.

This is certainly a micro-optimization, but on large sites every little bit can help. Thanks for the suggestion @bswatson!