Make WP_Automatic_Upgrader a proper object that gets instantiated. Renames nearly all of its methods.

Also renames wp_auto_updates_maybe_update() to wp_maybe_auto_update().

see #22704.


git-svn-id: https://develop.svn.wordpress.org/trunk@25823 602fd350-edb4-49c9-b593-d223f7449a82