Delete oEmbed caches on pre_post_update so cached meta is used

[nacin]

WP_Embed::delete_oembed_caches() is hooked into 'save_post', after which it calls get_post_meta() in a roundabout way. This results in an automatic update_meta_cache() call for a saved post, when it may not be otherwise needed for the rest of that pageload.

By moving oEmbed deletion to pre_post_update, we're able to use metadata that is more likely to already be in cache. If it is not already in cache but we do need metadata after, this could potentially add a query. But, I think the tradeoff is worth it.

Parent ticket: #22189. Saving navigation menus are slow.

[nacin]

In 22369:

Invalidate oEmbed caches on pre_post_update instead of save_post so existing metadata caches may be used. fixes #22190.