graph/db: move Topology client management to ChannelGraph

We plan to later on add an option for a remote graph source which will
be managed from the ChannelGraph. In such a set-up, a node would rely on
the remote graph source for graph updates instead of from gossip sync.
In this scenario, however, our topology subscription logic should still
notify clients of all updates and so it makes more sense to have the
logic as part of the ChannelGraph so that we can send updates we receive
from the remote graph.

rpcserver.go

@@ -48,7 +48,6 @@ import (
 	"github.com/lightningnetwork/lnd/feature"
 	"github.com/lightningnetwork/lnd/fn/v2"
 	"github.com/lightningnetwork/lnd/funding"
-	"github.com/lightningnetwork/lnd/graph"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/htlcswitch"
@@ -3347,7 +3346,7 @@ func (r *rpcServer) GetInfo(_ context.Context,
 	// TODO(roasbeef): add synced height n stuff
 
 	isTestNet := chainreg.IsTestnet(&r.cfg.ActiveNetParams)
-	nodeColor := graph.EncodeHexColor(nodeAnn.RGBColor)
+	nodeColor := graphdb.EncodeHexColor(nodeAnn.RGBColor)
 	version := build.Version() + " commit=" + build.Commit
 
 	return &lnrpc.GetInfoResponse{
@@ -6960,7 +6959,7 @@ func marshalNode(node *models.LightningNode) *lnrpc.LightningNode {
 		PubKey:        hex.EncodeToString(node.PubKeyBytes[:]),
 		Addresses:     nodeAddrs,
 		Alias:         node.Alias,
-		Color:         graph.EncodeHexColor(node.Color),
+		Color:         graphdb.EncodeHexColor(node.Color),
 		Features:      features,
 		CustomRecords: customRecords,
 	}
@@ -7158,7 +7157,7 @@ func (r *rpcServer) SubscribeChannelGraph(req *lnrpc.GraphTopologySubscription,
 
 	// First, we start by subscribing to a new intent to receive
 	// notifications from the channel router.
-	client, err := r.server.graphBuilder.SubscribeTopology()
+	client, err := r.server.graphDB.SubscribeTopology()
 	if err != nil {
 		return err
 	}
@@ -7211,7 +7210,7 @@ func (r *rpcServer) SubscribeChannelGraph(req *lnrpc.GraphTopologySubscription,
 // returned by the router to the form of notifications expected by the current
 // gRPC service.
 func marshallTopologyChange(
-	topChange *graph.TopologyChange) *lnrpc.GraphTopologyUpdate {
+	topChange *graphdb.TopologyChange) *lnrpc.GraphTopologyUpdate {
 
 	// encodeKey is a simple helper function that converts a live public
 	// key into a hex-encoded version of the compressed serialization for