Revisions: optimize performance when post has large number of revisions.

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.


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