“Gunpowder Milkshake” is the latest in a long line of action films about double-crossed hitmen on the run from their retainers, from “Le Samourai” to “Branded to Kill” to “In Bruges”; it’s also the latest attempt at a “John Wick” riff, and a riff of poliziotteschi movies, and a whole bunch of other things at the same time.