add card dip UUID so identical cards always look different

[benadida]

Problem: if two cards are identical and are swapped at just the right time, they will look like the same card to a consumer of this module. That's no good.

Solution: every time a card is dipped and read, this module should assign a random UUID identifying this "card dip instance" and include it in the result value of a /get. That way, if a card is removed and reinserted, or replaced with an identical card, the consumer can make a decision about what to do, as opposed to assuming the card never moved.