Messages are now tagged with the IdentityKey fingerprint of the
originating session. IdentityKeys have one of three trust states:
undecided (default), trusted, and untrusted/not yet trusted.
Previously, the sender was assumed to be the conversation counterpart.
This broke carboned own-device messages. We now track the sender
properly, and also set the status (sent by one of the own devices vs
received from the counterpart) accordingly.
