Introduce metadata_exists(), WP_User::get_data_by(), WP_User::get(),...

Introduce metadata_exists(), WP_User::get_data_by(), WP_User::get(), WP_User::has_prop().  Don't fill user objects with meta.  Eliminate data duplication in cache and memory.  Props scribu. see #15458

git-svn-id: https://core.svn.wordpress.org/trunk@18597 1a063a9b-81f0-0310-95a4-ce76da25c4cd