Users: Cache database queries within WP_User_Query class.
Cache the results of database queries within WP_User_Query class. Only cache queries that are requesting 3 or less fields so that caches are not storing full user objects. Cache results are stored in a new global cache group named users-queries. Add a new parameter to WP_User_Query called cache_results to allow developers to opt out of a receiving cached results. cache_results parameter defaults to true. Also add a new helper function called wp_cache_set_users_last_changed, similar to wp_cache_set_posts_last_changed that incroments last changed value in cache group users. Ensure that wp_cache_set_users_last_changed is called whenever user / user meta is modified for proper cache invalidation.
Props johnjamesjacoby, spacedmonkey, westi, dd32, strategio, srikanthmeenakshi, OllieJones, khoipro, rjasdfiii, flixos90, mukesh27, peterwilsoncc.
Fixes #40613.