Fix EnforcingChannelKeys panic when our counterparty burns their $.

Fix EnforcingChannelKeys panic when our counterparty burns their $.

If our counterparty burns their funds by revoking their current
commitment transaction before we've sent them a new one, we'll step
forward the remote commitment number. This would be otherwise fine
(and may even encourage them to broadcast their revoked state(s) on
chain), except that our new EnforcingChannelKeys expects us to not
jump forward in time. Since it isn't too important that we punish
our counterparty in such a corner-case, we opt to just close the
channel in such a case and move on.