Use nounce for replay protection

[rugk]

From the Threema whitepaper:

Replay Prevention
The Threema app remembers the nonce of every message that has been sent in the past, and rejects messages with duplicate nonces. Since the server cannot successfully modify the nonce of a message without knowing the private key of one of the parties involved in the communication, this prevents a malicious server from replaying/duplicating previously sent messages.