LONDON (Reuters) - Central banks may be about to face one of their biggest challenges yet: Global economic growth is slowing, but they have struggled to build up their policy arsenals since the last crisis to effectively deal with another one. FILE PHOTO: The Federal Reserve building is pictured in Washington, DC, U.S., August 22, 2018. REUTERS/Chris Wattie The U.S.-China trade war, worries about Britain’s imminent departure from the European Union, a potential recession in Italy and unrest...