Add state

[keks]

Inbome-aware clients need to keep some state. For each email-address the following information needs to be stored:

• time when the last email was received
• the last seen key
• whether they want to receive encrypted mail
• maybe some other stuff ???

@dkg you were presenting the results of that session and I can't find docs anywhere. Do you still have that table of stuff that needs to be stored?