Add cache invalidation when updating a term, example: create a category, assign it to a post, edit the category. Currently, the post's term cache is not updated. When updating terms in a given taxonomy, invalidate the object term caches linked to that taxonomy. 

Introduce `get_taxonomy_last_changed()`, `set_taxonomy_last_changed()`, and `post_taxonomy_is_fresh()`.

`post_taxonomy_is_fresh()` is only called in `get_object_term_cache()` - at which point the taxonomy's `last_changed` value is checked against the post's `{$taxonomy}_last_changed` value.

`set_taxonomy_last_changed()` is called whenever directory database queries are made that insert new terms or affect existing terms.

Fixes #22526.



git-svn-id: https://develop.svn.wordpress.org/trunk@27101 602fd350-edb4-49c9-b593-d223f7449a82