The biggest mystery about Record Store Day used to be “What’s Record Store Day?” That question was answered a decade ago when a small group of retailers fought back against the death of brick-and-mortar music sales at the hands of digital disruptors. Now, an international alliance of mom-and-pop shops and indie chains offer RSD (as it’s called) exclusive releases and other incentives to bring customers through the door. For this year’s Record Store Day — returning Saturday, April 21 — the biggest questions are: “What can I get?