Cache coin proofs in local storage

[aupiff]

@gakonst writes:

Regarding the p2p thing, we can implement it so that when a client receives a coin, they automatically poll the operator for the needed proofs for that coin, and validate the proofs and the coin's history. If they all check out, the receiver accepts the coin and saves the proofs for later in local storage, in case they want to exit the coin. Otherwise they reject the transaction. When they send the coin to another user, they no longer need to hold the proofs so they can safely delete it from their local storage.