padDiff.js: convert to Promises/async
padDiff.js: convert to Promises/async

joequant/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite