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.

git-svn-id: https://develop.svn.wordpress.org/trunk@34529 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -266,7 +266,7 @@ class wpdb {
 	 * @var array
 	 */
 	var $tables = array( 'posts', 'comments', 'links', 'options', 'postmeta',
-		'terms', 'term_taxonomy', 'term_relationships', 'commentmeta' );
+		'terms', 'term_taxonomy', 'term_relationships', 'termmeta', 'commentmeta' );
 
 	/**
 	 * List of deprecated WordPress tables
@@ -382,6 +382,15 @@ class wpdb {
 	 */
 	public $term_taxonomy;
 
+	/**
+	 * WordPress Term Meta table.
+	 *
+	 * @since 4.4.0
+	 * @access public
+	 * @var string
+	 */
+	public $termmeta;
+
 	/*
 	 * Global and Multisite tables
 	 */