multisite: WP_User compatibility keys inacessible after switch_to_blog

[flaviozavan]

Some WP_User attributes are accessed using $back_compat_keys and get_user_meta:

https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-user.php#L121
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-user.php#L312

$back_compat_keys is static and defined when the first WP_User is instantiated.
This process hardcodes $wpdb->prefix into some keys (e.g. user_level), preventing it from functioning correctly after switch_to_blog().

Additionally, unsetting any of these attributes for one user will unset it for all Users, due to $back_compat_keys being static:
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-user.php#L374