This change took me a while to work out. I see what you're doing now, but I think it is incomplete.

I think it would be good to:

* in worker.js make `spa.on("offer", ...` act like `spa.on("message"` in the sequence of creating the `Conversation` object, then calling `Conversation#handleIncomingCall`.
* That would then allow:
** `Conversation` not to store the offer
** No need to do complex checking for the store talkilla.conversation-incoming message - just send the existing queue

I don't mind if this is done in a follow-up PR, but I think it should address the debt before we move on.