[racket] Rosetta: Atomic updates task

[Daniel Prager]

I've added my Racket solution to the Atomic Updates task.  See

    http://rosettacode.org/wiki/Atomic_updates

I'd be interested in suggestions on how to improve this solution,
especially by drawing on existing facilities in Racket to construct a more
elegant approach (or approaches) than the hand-rolled locks that I employed.

-- Dan

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