graph+server: add template for new graph Builder sub-system

This is preparation for an upcoming commit that will move over various
responsibilities from the ChannelRouter to the graph Builder. So that
that commit can be a pure code-move commit, the template for the new
sub-system is added up front here.

server.go

@@ -41,6 +41,7 @@ import (
 	"github.com/lightningnetwork/lnd/feature"
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -271,6 +272,8 @@ type server struct {
 
 	missionControl *routing.MissionControl
 
+	graphBuilder *graph.Builder
+
 	chanRouter *routing.ChannelRouter
 
 	controlTower routing.ControlTower
@@ -973,6 +976,11 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	strictPruning := cfg.Bitcoin.Node == "neutrino" ||
 		cfg.Routing.StrictZombiePruning
 
+	s.graphBuilder, err = graph.NewBuilder(&graph.Config{})
+	if err != nil {
+		return nil, fmt.Errorf("can't create graph builder: %w", err)
+	}
+
 	s.chanRouter, err = routing.New(routing.Config{
 		SelfNode:            selfNode.PubKeyBytes,
 		RoutingGraph:        graphsession.NewRoutingGraph(chanGraph),
@@ -2019,6 +2027,12 @@ func (s *server) Start() error {
 		}
 		cleanup = cleanup.add(s.authGossiper.Stop)
 
+		if err := s.graphBuilder.Start(); err != nil {
+			startErr = err
+			return
+		}
+		cleanup = cleanup.add(s.graphBuilder.Stop)
+
 		if err := s.chanRouter.Start(); err != nil {
 			startErr = err
 			return