Turn delete_meta() , get_post_meta_by_id(), update_meta(),...

Turn delete_meta() , get_post_meta_by_id(), update_meta(), delete_post_meta_by_key() into wrappers around the metadata API. Add back compat *_postmeta actions to metadata API. Props jgadbois. see #18196

git-svn-id: https://core.svn.wordpress.org/trunk@18500 1a063a9b-81f0-0310-95a4-ce76da25c4cd