Adherents to Ayn Rand’s philosophy and writings are generating record sales for local businessman Harold Mercer, owner of Who Is John Malt?, a milkshake shop opened last month in downtown Boston. Mercer’s milkshakes, which do not contain any dairy, and consist primarily of chilled, blended organic fruits and juices, are intensely debated within the frozen beverage community, with supporters defending the entrepreneur’s radical vision, and detractors questioning whether his product semantically qualifies as a milkshake at all.