The Shrub-a-Dub-Dub cocktail (created by Kate Gerwin) is damn delicious . . . and I’m pretty sure the strawberry-rhubarb shrub has a lot do with it. Unsure what I’m talking about when I say shrub? Think Shaker & Spoon got a little Willy Wonka on some bushes? Maybe I should clarify: the cocktail pros at Shaker & Spoon didn’t invent a hybrid strawberry-rhubarb…