Multisite: Change WP_Network id property to an integer.

For consistency and developer sanity.

Props flixos90.
Fixes #37050.

Built from https://develop.svn.wordpress.org/trunk@37870


git-svn-id: http://core.svn.wordpress.org/trunk@37811 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-settings.php

@@ -99,6 +99,7 @@ require( ABSPATH . WPINC . '/class-wp-error.php' );
 require( ABSPATH . WPINC . '/pomo/mo.php' );
 
 // Include the wpdb class and, if present, a db.php database drop-in.
+global $wpdb;
 require_wp_db();
 
 // Set the database table prefix and the format specifiers for database table columns.