contractcourt: make synchronous chain watcher notifications optional

In this commit, we modify the way that notifications are dispatched
within the chainWatcher. Before we would *always* wait for an ack back
before we started to clean up he database state. This would at times
lead to deadlocks. To remedy this, we now allow callers to decide if
they want notifications to be sync or not. The only current caller that
requires this is the breach arbiter.

server.go

@@ -417,11 +417,16 @@ func newServer(listenAddrs []string, chanDB *channeldb.DB, cc *chainControl,
 		GenSweepScript: func() ([]byte, error) {
 			return newSweepPkScript(cc.wallet)
 		},
-		Notifier:               cc.chainNotifier,
-		PublishTransaction:     cc.wallet.PublishTransaction,
-		SubscribeChannelEvents: s.chainArb.SubscribeChannelEvents,
-		Signer:                 cc.wallet.Cfg.Signer,
-		Store:                  newRetributionStore(chanDB),
+		Notifier:           cc.chainNotifier,
+		PublishTransaction: cc.wallet.PublishTransaction,
+		SubscribeChannelEvents: func(chanPoint wire.OutPoint) (*contractcourt.ChainEventSubscription, error) {
+			// We'll request a sync dispatch to ensure that the channel
+			// is only marked as closed *after* we update our internal
+			// state.
+			return s.chainArb.SubscribeChannelEvents(chanPoint, true)
+		},
+		Signer: cc.wallet.Cfg.Signer,
+		Store:  newRetributionStore(chanDB),
 	})
 
 	// Create the connection manager which will be responsible for