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/version.php

@@ -11,7 +11,7 @@ $wp_version = '4.4-alpha-33636-src';
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 34030;
+$wp_db_version = 34528;
 
 /**
  * Holds the TinyMCE version