It’s been 40 years since Boz Scaggs released his breakthrough album, “Silk Degrees,” which spawned the hits “Lido Shuffle,” “It’s Over,” “What Can I Say,” and the Grammy-winning blues track “Lowdown.” And while he laid low for most of the 1980s, since 1994 he’s been plying his signature mesh of pop, R&B and soul on albums that have topped the Billboard jazz and blues charts.