This winter, four places showcasing cocktails opened in Boston at essentially the same time: the Hawthorne, Brick & Mortar, Saloon, and backbar. The city has turned into a hub for craft cocktails - those made with fresh, high-quality ingredients, often reviving recipes from decades and centuries past. It’s easy to forget that until fairly recently drinks were made with neon “sour mix’’ and bitters were nearly impossible to find. How did Boston become such a cocktail town?