Query: Cache post ID database query within WP_Query.
Add object caching to the first database query in WP_Query, ie the database query for post IDs that match the desired result. Randomly ordered queries remain uncached as they are not intended to return consistent results.
Caching of ID queries is enabled by default, per the post object, term and meta caches.
Props spacedmonkey, aaroncampbell, batmoo, chriscct7, costdev, dd32, drewapicture, johnbillion, mukesh27, nacin, ocean90, peterwilsoncc, ryan, scribu, sergeybiryukov, tillkruss.
Fixes #22176.