contractcourt/channel_arbitrator: handle breach channel at startup

Earlier the channel arbitrator would fail to recognize channels pending
close that were in the breached state. This lead to the state machine
not progressing correctly, and in some cases crashing since we would
attempt to force close an already closed channel.

A test TestChannelArbitratorForceCloseBreachedChannel is added to
exercise one of these scenarios.

contractcourt/channel_arbitrator.go

@@ -357,6 +357,9 @@ func (c *ChannelArbitrator) Start() error {
 			case channeldb.CooperativeClose:
 				trigger = coopCloseTrigger
 
+			case channeldb.BreachClose:
+				trigger = breachCloseTrigger
+
 			case channeldb.LocalForceClose:
 				trigger = localCloseTrigger