Multisite: More specific caching for get_dirsize.

Instead of one cache entry for all upload folders for a site on multisite, this now caches for each folder and invalidates that cache based on context. In multisite, this should speed up `get_dirsize` calls since older directories that are much less likely to change will no longer have the size recalculated.

Props janthiel, A5hleyRich, batmoo.
Fixes #19879.

Built from https://develop.svn.wordpress.org/trunk@49212


git-svn-id: http://core.svn.wordpress.org/trunk@48974 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/file.php

@@ -928,7 +928,7 @@ function _wp_handle_upload( &$file, $overrides, $time, $action ) {
 	$url = $uploads['url'] . "/$filename";
 
 	if ( is_multisite() ) {
-		delete_transient( 'dirsize_cache' );
+		invalidate_dirsize_cache( $new_file );
 	}
 
 	/**