multi: add abstraction for Router and SessionSource graph access

In this commit, we completely remove the Router's dependence on a Graph
source that requires a `kvdb.RTx`. In so doing, we are more prepared for
a future where the Graph source is backed by different DB structure such
as pure SQL.

The two areas affected here are: the ChannelRouter's graph access that
it uses for pathfinding. And the SessionSource's graph access that it
uses for payments.

The ChannelRouter gets given a Graph and the SessionSource is given a
GraphSessionFactory which it can use to create a new session. Behind the
scenes, this will acquire a kvdb.RTx that will be used for calls to the
Graph's `ForEachNodeChannel` method.

server.go

@@ -32,6 +32,7 @@ import (
 	"github.com/lightningnetwork/lnd/chanbackup"
 	"github.com/lightningnetwork/lnd/chanfitness"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channeldb/graphsession"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/clock"
@@ -956,7 +957,9 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		return nil, fmt.Errorf("error getting source node: %w", err)
 	}
 	paymentSessionSource := &routing.SessionSource{
-		Graph:             chanGraph,
+		GraphSessionFactory: graphsession.NewGraphSessionFactory(
+			chanGraph,
+		),
 		SourceNode:        sourceNode,
 		MissionControl:    s.missionControl,
 		GetLink:           s.htlcSwitch.GetLinkByShortID,
@@ -967,9 +970,11 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.controlTower = routing.NewControlTower(paymentControl)
 
-	strictPruning := (cfg.Bitcoin.Node == "neutrino" ||
-		cfg.Routing.StrictZombiePruning)
+	strictPruning := cfg.Bitcoin.Node == "neutrino" ||
+		cfg.Routing.StrictZombiePruning
+
 	s.chanRouter, err = routing.New(routing.Config{
+		RoutingGraph:        graphsession.NewRoutingGraph(chanGraph),
 		Graph:               chanGraph,
 		Chain:               cc.ChainIO,
 		ChainView:           cc.ChainView,