graph+lnd: add various calls to GraphSource

In this commit, we add a bunch of graph methods to the GraphSource, let
DBSource implement it and then we use the graph source for these
methods for the GetNodeInfo, VerifyMessage, DescribeGraph,
GetNodeMetrics, GetChanInfo and GetNodeInfo RPC calls along with peer
alias lookup.

server.go

@@ -4962,12 +4962,14 @@ var errNoAdvertisedAddr = errors.New("no advertised address found")
 
 // fetchNodeAdvertisedAddrs attempts to fetch the advertised addresses of a node.
 func (s *server) fetchNodeAdvertisedAddrs(pub *btcec.PublicKey) ([]net.Addr, error) {
+	ctx := context.TODO()
+
 	vertex, err := route.NewVertexFromBytes(pub.SerializeCompressed())
 	if err != nil {
 		return nil, err
 	}
 
-	node, err := s.graphDB.FetchLightningNode(vertex)
+	node, err := s.graphSource.FetchLightningNode(ctx, vertex)
 	if err != nil {
 		return nil, err
 	}