Embeds: Improve consistency of update and refresh logic for oEmbed caching...

Embeds: Improve consistency of update and refresh logic for oEmbed caching between `oembed_cache` and post meta.

* Allow updating oEmbed cache during `parse-embed` requests for non-post editors (such as widgets).
* Update any existing `oembed_cache` post when `usecache` and TTL has passed.
* Do not overwrite a previously valid cache with `{{unknown}}`.

Props dlh.
See #34115.
Fixes #42310.

Built from https://develop.svn.wordpress.org/trunk@42009


git-svn-id: https://core.svn.wordpress.org/trunk@41843 1a063a9b-81f0-0310-95a4-ce76da25c4cd