Assure freshness of contact verification attempts

[azul]

Bryan Ford is concerned with potential replay attacks a MITM attacker might be able to pull off against either Alice or Bob.

Adding a timestamp or expiry to ensure freshness would mitigate these.

(This issue is part of the feedback on the messaging@moderncrypto mailing list #61)

[azul]

The most simple approach to me seems to track the time a QR code was shown or scanned on the respective device. The invite would remain valid for one day. If no message is received in that timeframe the record is removed from the database. This way we would not need to transfer timestamps or worry about synchronized clocks and timezones etc.

[azul]

#66 addressed this.