It's useful to expose this bit of state because, although callers can
keep track of it themselves, it's already in the Conversation so it
seems wasteful to make them do so.
R=golang-dev, ioerror
CC=golang-dev
https://golang.org/cl/6724056
The explicit OTR termination signal tells the peer that they should
discard their session keys and, more importantly, tells them that
*you* have done so and therefore cannot decrypt future messages.
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6430043
Adam Langley
Shenghou Ma