sync sent private messages between Twister instances

(see issue #164)
@dryabov isn't this printf going to be triggered for every DM? 
i mean, let's say A sends dm to B. I'm C. 
In the first loop above we try decrypting it with every privkey we have (C), but it will fail silently. ok.
But since the dm->s is encrypted to "A" and we don't have this key in our wallet this will cause this printf to display, no? maybe i'm missing something.