multi: add Start and Stop methods for ChannelGraph

We do this in preparation for moving channel cache population logic out
of the constructor and into the Start method. We also will later on
(when topology subscription is moved to the ChannelGraph), have a
goroutine that will need to be kicked off and stopped.

server.go

@@ -2377,6 +2377,12 @@ func (s *server) Start() error {
 			return
 		}
 
+		cleanup = cleanup.add(s.graphDB.Stop)
+		if err := s.graphDB.Start(); err != nil {
+			startErr = err
+			return
+		}
+
 		cleanup = cleanup.add(s.graphBuilder.Stop)
 		if err := s.graphBuilder.Start(); err != nil {
 			startErr = err
@@ -2683,6 +2689,9 @@ func (s *server) Stop() error {
 		if err := s.graphBuilder.Stop(); err != nil {
 			srvrLog.Warnf("failed to stop graphBuilder %v", err)
 		}
+		if err := s.graphDB.Stop(); err != nil {
+			srvrLog.Warnf("failed to stop graphDB %v", err)
+		}
 		if err := s.chainArb.Stop(); err != nil {
 			srvrLog.Warnf("failed to stop chainArb: %v", err)
 		}