Roll back if sub-channel is stuck in Offered or CloseOffered

[luckysori]

We would like to be able to roll back to a previous state if a sub-channel protocol cannot make progress for whatever reason and it is safe to forget about all the information that has been exchanged thus far.

One such instance is after a channel update has been offered (Offered and CloseOffered states). Once #116 is merged I think the Accepted state can also be rolled back since we no longer send the new split transaction signature with it. I don't think we can treat CloseAccepted the same way because the SubChannelCloseAccept message already includes the new commitment transaction signature.

[luckysori]

This has come up recently again whilst working on 10101. What do you think, @Tibo-lg?

[Tibo-lg]

For the "regular" dlc channels there is a timeout mechanism, which IIRC simply force closes the channel after some time if the peer doesn't answer a message in a timely manner. Rolling back could also work I guess, I don't have a strong opinion, but maybe take a look at the implementation for the dlc channels and see if you think that would fit with what you had in mind.