Send acknowledgement messages to keep keys fresh

[fabiocolacio]

A user only creates a new private key every time he sends a message, because he must send the public key to his peer.

This means that if one user sends many consecutive messages to another, only the sender will be creating new DH parameters, and the other user's DH parameters will be reused. In order to minimize the reuse of DH information, acknowledgement messages can be incorporated into the protocol.

Every time a user receives a message, that user will create a new pair of DH parameters. He will send a message with an empty body (which is not visible to the users) and the public key, to acknowledge that he received the previous message, and advertise his new key.

This also adds the possibility to add "message read" notifications to users.