By Rajesh Kumar Singh CHICAGO (Reuters) – Persistent staffing shortages, fewer flights and booming demand have cast a shadow on the busy July Fourth holiday weekend and the entire summer travel season. While the lifting of COVID-19 curbs and bottled-up travel demand are translating into the strongest summer since the pandemic for American carriers, frequent mass flight cancellations are creating chaos for their customers. U.S. airlines have canceled more than 21,000 flights, or about 2.7% of the scheduled total since the Memorial Day holiday weekend in May, almost double last year’s rate, acco…