North Korea Tops Powell's Agenda in Asia   Four years ago this weekend, then-Secretary of State Madeleine K. Albright visited North Korea, in what appeared to be a breakthrough in U.S. relations with the reclusive Stalinist government. Today, Secretary of State Colin L. Powell leaves on what may be his last official visit to East Asia -- hopping through Japan, China and South Korea over three days -- with the crisis over North Korea's nuclear ambitions at the top of his agenda.