Introduce metadata for taxonomy terms.
Adds a new table to the database schema (wp_termmeta), and a set of
*_term_meta() API functions. get_terms() and wp_get_object_terms()
now also support 'meta_query' parameters, with syntax identical to other
uses of WP_Meta_Query.
When fetching terms via get_terms() or wp_get_object_terms(), metadata for
matched terms is preloaded into the cache by default. Disable this behavior
by setting the new $update_term_meta_cache paramater to false.
To maximize performance, within WP_Query loops, the termmeta cache is *not*
primed by default. Instead, we use a lazy-loading technique: metadata for all
terms belonging to posts in the loop is loaded into the cache the first time
that get_term_meta() is called within the loop.
Props boonebgorges, sirzooro.
See #10142.