Widget Posts: Do batch up-front widget_instance post retrieval

[westonruter]

Consider doing batch widget_instance post retrieval via a post__in. This should be done when the Customizer is going to get all registered widget settings, and it should be done for all widget IDs in wp_get_sidebar_widgets(). This is less of a concern when an external object cache is used, since in that case each widget will only get a DB call when first loading the page (e.g. SELECT * FROM wp_posts WHERE ID = 1999 LIMIT 1).