Regenerate the term hierarchy cache (`{taxonomy}_children`) when it is out of sync with the passed taxonomy's `last_changed` value.

Introduces `taxonomy_hierarchy_is_fresh()`, which is only called in `_get_term_hierarchy()`. The taxonomy's `last_changed` value is checked against the value of `wp_cache_get( 'hierarchy_last_changed', $taxonomy )`.

Adds a unit test - `Tests_Term:test_hierachy_invalidation()`.

See [27101], which makes this type of cache invalidation possible.
Fixes #14485.



git-svn-id: https://develop.svn.wordpress.org/trunk@27102 602fd350-edb4-49c9-b593-d223f7449a82