BALLOT_BOX: remove records, add receipts
A record (i.e. who voted, when and with which credential in the context of the web server) is a notion that does not exist in the cryptographic protocol and does not really make sense in the context of the command-line tool. By the way, we add the notion of receipt which, in the context of the web server, is the ballot hash computed by the server. Bonus: when a ballot is accepted, we compute its hash only once.