ANTHONY DEUTSCH IN normal times, Dutch disc jockey Oliver Heldens performs for thousands of fans at electronic dance shows around the world. But with festivals cancelled due to the coronavirus pandemic, the 25-year-old has spent most of 2020 in lockdown producing music and streaming sets online. Heldens, ranked by DJ Mag as the world's No. 8 for 2020, is one of a growing number of Dutch DJs to become household names. The global dance music industry is worth billions of dollars annually, including concerts, royalties and equipment. An estimated half a million people tuned in to Heldens' set before the…