More performance improvements to metadata lazyloading.

Comment and term meta lazyloading for `WP_Query` loops, introduced in 4.4,
depended on filter callback methods belonging to `WP_Query` objects. This meant
storing `WP_Query` objects in the `$wp_filter` global (via `add_filter()`),
requiring that PHP retain the objects in memory, even when the local variables
would typically be expunged during normal garbage collection. In cases where a
large number of `WP_Query` objects were instantiated on a single pageload,
and/or where the contents of the `WP_Query` objects were quite large, serious
performance issues could result.

We skirt this problem by moving metadata lazyloading out of `WP_Query`. The
new `WP_Metadata_Lazyloader` class acts as a lazyload queue. Query instances
register items whose metadata should be lazyloaded - such as post terms, or
comments - and a `WP_Metadata_Lazyloader` method will intercept comment and
term meta requests to perform the cache priming. Since `WP_Metadata_Lazyloader`
instances are far smaller than `WP_Query` (containing only object IDs), and
clean up after themselves far better than the previous `WP_Query` methods (bp
only running their callbacks a single time for a given set of queued objects),
the resource use is decreased dramatically.

See [36525] for an earlier step in this direction.

Props lpawlik, stevegrunwell, boonebgorges.
Fixes #35816.

git-svn-id: https://develop.svn.wordpress.org/trunk@36566 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/meta.php

@@ -851,6 +851,23 @@ function update_meta_cache($meta_type, $object_ids) {
 	return $cache;
 }
 
+/**
+ * Get the metadata lazyloading queue.
+ *
+ * @since 4.5.0
+ *
+ * @return WP_Metadata_Lazyloader $lazyloader Metadata lazyloader queue.
+ */
+function wp_metadata_lazyloader() {
+	static $wp_metadata_lazyloader;
+
+	if ( null === $wp_metadata_lazyloader ) {
+		$wp_metadata_lazyloader = new WP_Metadata_Lazyloader();
+	}
+
+	return $wp_metadata_lazyloader;
+}
+
 /**
  * Given a meta query, generates SQL clauses to be appended to a main query.
  *