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.

peer.go

@@ -370,7 +370,9 @@ func (p *peer) loadActiveChannels(chans []*channeldb.OpenChannel) error {
 		// Register this new channel link with the HTLC Switch. This is
 		// necessary to properly route multi-hop payments, and forward
 		// new payments triggered by RPC clients.
-		chainEvents, err := p.server.chainArb.SubscribeChannelEvents(*chanPoint)
+		chainEvents, err := p.server.chainArb.SubscribeChannelEvents(
+			*chanPoint, false,
+		)
 		if err != nil {
 			return err
 		}
@@ -1259,7 +1261,9 @@ out:
 				peerLog.Errorf("unable to get best block: %v", err)
 				continue
 			}
-			chainEvents, err := p.server.chainArb.SubscribeChannelEvents(*chanPoint)
+			chainEvents, err := p.server.chainArb.SubscribeChannelEvents(
+				*chanPoint, false,
+			)
 			if err != nil {
 				peerLog.Errorf("unable to subscribe to chain "+
 					"events: %v", err)