PARIS, May 18 (Reuters) - The U.S. Treasury decided to extend its sanctions waiver on Russian seaborne oil, which lapsed on Saturday, after several countries asked for more time to buy Russian oil, a source familiar with the plan said on Monday. The waiver will last another 30 days, the source said. The Reuters Power Up newsletter provides everything you need to know about the global energy industry. Sign up here. The United States had issued the waiver in a bid to ease oil supply shortages and high prices due to Iran's closure of the Strait of Hormuz amid a...