lnwallet+peer: add new ResetState method to channel state machine

In this commit, we add a new ResetState method to the channel state
machine which will reset the state of the channel to `channelOpen`. We
add this as before this commit, it was possible for a channel to shift
into the closing state, the closing negotiation be cancelled for
whatever reason, resulting the the channel held by the breachArbiter
unable to act to potential on-chain events.

lnwallet/channel.go

@@ -1382,6 +1382,13 @@ func (lc *LightningChannel) CancelObserver() {
 	close(lc.observerQuit)
 }
 
+// ResetState resets the state of the channel back to the default state. This
+// ensures that any active goroutines which need to act based on on-chain
+// events do so properly.
+func (lc *LightningChannel) ResetState() {
+	lc.Lock()
+	lc.status = channelOpen
+	lc.Unlock()
 }
 
 // logUpdateToPayDesc converts a LogUpdate into a matching PaymentDescriptor