Maintain a cache of idempotency keys and check for reuse

[brandur-stripe]

It would be nice if stripe-mock maintained a set of recently used idempotency keys to detect when a client tries to reuse a key with different parameters. In those cases we could reject the request just like the real Stripe API would do. (Keys would be evicted from the cache after some period of time so that it doesn't grow in an unbounded way.)

This would have caught the problem in stripe/stripe-ruby#598.