Idempotent Balance Transfers

Process balance transfers with idempotency keys, the way a payments API protects against clients retrying a request.

initial maps account ids to starting balances. Each operation is [key, sender, receiver, amount], processed in order:

• New key: if the sender's balance is at least amount, move amount from sender to receiver and the result is "applied". Otherwise nothing moves and the result is "insufficient_funds". Either way, the key now remembers its parameters (sender, receiver, amount) and its result.
• Seen key, identical parameters: a retry. Nothing moves, and the result is the key's original result (even if the balance has changed since).
• Seen key, different parameters: nothing moves, the result is "key_conflict", and the key keeps its original parameters and result.

An account missing from initial has balance 0; a receiver that gets money is added to the balances.

Return [results, balances], where results has one string per operation in order and balances is a list of [account, balance] for every account in initial plus every account that received money, sorted by account id.