Multisite: Introduce WP_Site_Query

Provides a consistent way to query `$wpdb->blogs` for `WP_Site` objects based on domain, path, site ID, network ID, and more.

Introduces and uses `update_site_cache()` and `_prime_site_caches()` to maintain a cached list of `WP_Site` objects for use in multiple queries.

Props spacedmonkey, flixos90, DrewAPicture, jeremyfelt, ocean90.
See #35791.


git-svn-id: https://develop.svn.wordpress.org/trunk@37477 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/date.php

@@ -491,7 +491,7 @@ class WP_Date_Query {
 		$valid_columns = array(
 			'post_date', 'post_date_gmt', 'post_modified',
 			'post_modified_gmt', 'comment_date', 'comment_date_gmt',
-			'user_registered',
+			'user_registered', 'registered', 'last_updated',
 		);
 
 		// Attempt to detect a table prefix.
@@ -525,6 +525,10 @@ class WP_Date_Query {
 				$wpdb->users => array(
 					'user_registered',
 				),
+				$wpdb->blogs => array(
+					'registered',
+					'last_updated',
+				),
 			);
 
 			// If it's a known column name, add the appropriate table prefix.