Always include wp-db.php. Prevents a conditional include and allows db dropins...

Always include wp-db.php. Prevents a conditional include and allows db dropins to cleanly extend the wpdb class. Move require_wp_db() to load.php for consistency with bootloader helpers. fixes #14508.

git-svn-id: https://core.svn.wordpress.org/trunk@15638 1a063a9b-81f0-0310-95a4-ce76da25c4cd