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.
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