multi: remove the need for NewRoutingGraph

The `graphsession.NewRoutingGraph` method was used to create a
RoutingGraph instance with no consistent read transaction across calls.
But now that the ChannelGraph directly implements this, we can remove
The NewRoutingGraph method.

graph/db/graph.go

@@ -594,6 +594,30 @@ func (c *ChannelGraph) FetchNodeFeaturesTx(tx kvdb.RTx,
 	}
 }
 
+// ForEachNodeDirectedChannel iterates through all channels of a given node,
+// executing the passed callback on the directed edge representing the channel
+// and its incoming policy. If the callback returns an error, then the iteration
+// is halted with the error propagated back up to the caller. If the graphCache
+// is available, then it will be used to retrieve the node's channels instead
+// of the database.
+//
+// Unknown policies are passed into the callback as nil values.
+func (c *ChannelGraph) ForEachNodeDirectedChannel(nodePub route.Vertex,
+	cb func(channel *DirectedChannel) error) error {
+
+	return c.ForEachNodeDirectedChannelTx(nil, nodePub, cb)
+}
+
+// FetchNodeFeatures returns the features of the given node. If no features are
+// known for the node, an empty feature vector is returned.
+// If the graphCache is available, then it will be used to retrieve the node's
+// features instead of the database.
+func (c *ChannelGraph) FetchNodeFeatures(nodePub route.Vertex) (
+	*lnwire.FeatureVector, error) {
+
+	return c.FetchNodeFeaturesTx(nil, nodePub)
+}
+
 // ForEachNodeCached is similar to forEachNode, but it utilizes the channel
 // graph cache instead. Note that this doesn't return all the information the
 // regular forEachNode method does.

graph/graphsession/graph_session.go

@@ -56,15 +56,6 @@ type session struct {
 	tx    kvdb.RTx
 }
 
-// NewRoutingGraph constructs a session that which does not first start a
-// read-only transaction and so each call on the routing.Graph will create a
-// new transaction.
-func NewRoutingGraph(graph ReadOnlyGraph) routing.Graph {
-	return &session{
-		graph: graph,
-	}
-}
-
 // close closes the read-only transaction being used to access the backing
 // graph. If no transaction was started then this is a no-op.
 func (g *session) close() error {

rpcserver.go

@@ -51,7 +51,6 @@ import (
 	"github.com/lightningnetwork/lnd/graph"
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
-	"github.com/lightningnetwork/lnd/graph/graphsession"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
 	"github.com/lightningnetwork/lnd/input"
@@ -711,8 +710,8 @@ func (r *rpcServer) addDeps(s *server, macService *macaroons.Service,
 			amount lnwire.MilliSatoshi) (btcutil.Amount, error) {
 
 			return routing.FetchAmountPairCapacity(
-				graphsession.NewRoutingGraph(graph),
-				selfNode.PubKeyBytes, nodeFrom, nodeTo, amount,
+				graph, selfNode.PubKeyBytes, nodeFrom, nodeTo,
+				amount,
 			)
 		},
 		FetchChannelEndpoints: func(chanID uint64) (route.Vertex,

server.go

@@ -1073,7 +1073,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.chanRouter, err = routing.New(routing.Config{
 		SelfNode:           selfNode.PubKeyBytes,
-		RoutingGraph:       graphsession.NewRoutingGraph(dbs.GraphDB),
+		RoutingGraph:       dbs.GraphDB,
 		Chain:              cc.ChainIO,
 		Payer:              s.htlcSwitch,
 		Control:            s.controlTower,