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-settings.php

@@ -135,6 +135,7 @@ require( ABSPATH . WPINC . '/class-wp-user-query.php' );
 require( ABSPATH . WPINC . '/session.php' );
 require( ABSPATH . WPINC . '/meta.php' );
 require( ABSPATH . WPINC . '/class-wp-meta-query.php' );
+require( ABSPATH . WPINC . '/class-wp-metadata-lazyloader.php' );
 require( ABSPATH . WPINC . '/general-template.php' );
 require( ABSPATH . WPINC . '/link-template.php' );
 require( ABSPATH . WPINC . '/author-template.php' );