Improve speed and reduce the memory footprint when loading posts with many revisions.
* Use a direct query in `wp_get_post_autosave` to avoid loading all revisions.
* Query for IDs vs full objects in `register_and_do_post_meta_boxes`.
Props pdfernhout, johnnyb, miqrogroove, ocean90, senatorman, DBrumbaugh10Up, martijn-van-der-kooij, pavelevap, mackensen, mikeyarce, whyisjake.
Fixes#34560.
git-svn-id: https://develop.svn.wordpress.org/trunk@48422 602fd350-edb4-49c9-b593-d223f7449a82
4adb926ce8