Cache `get_term_by()` calls:

* Add a helper function, `wp_get_last_changed()`, to retrieve a last-modified timestamp by cache group
* When caching a term, also make cache entries for slug and name via `slug:{$term_id}` and `name:{$term_id}` keys in the `$taxonomy:$last_changed` bucket that reference the term_id
* In `clean_term_cache()` and `update_term_cache()`, respect `$_wp_suspend_cache_invalidation`
* Original term cache entries maintain BC

Adds unit tests.

Props wonderboymusic, tollmanz, boonebgorges.
Fixes #21760.


git-svn-id: https://develop.svn.wordpress.org/trunk@29915 602fd350-edb4-49c9-b593-d223f7449a82