By THÉRÈSE MARGOLIS     It wasn’t that long ago – about a decade or two, at most – that the mere mention of mezcal conjured up images of grungy-looking B. Traven-wannabes in seedy little cantinas in Oaxaca downing shots of agave rotgut and spring-breaker barroom daredevils testing their intestinal fortitude and sanity in sordid shacks on the squalid outskirts of the