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.

rpcserver.go

@@ -41,6 +41,7 @@ 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/contractcourt"
@@ -691,22 +692,9 @@ func (r *rpcServer) addDeps(s *server, macService *macaroons.Service,
 		FetchAmountPairCapacity: func(nodeFrom, nodeTo route.Vertex,
 			amount lnwire.MilliSatoshi) (btcutil.Amount, error) {
 
-			routingGraph, err := routing.NewCachedGraph(graph)
-			if err != nil {
-				return 0, err
-			}
-			defer func() {
-				closeErr := routingGraph.Close()
-				if closeErr != nil {
-					rpcsLog.Errorf("not able to close "+
-						"routing graph tx: %v",
-						closeErr)
-				}
-			}()
-
 			return routing.FetchAmountPairCapacity(
-				routingGraph, selfNode.PubKeyBytes, nodeFrom,
-				nodeTo, amount,
+				graphsession.NewRoutingGraph(graph),
+				selfNode.PubKeyBytes, nodeFrom, nodeTo, amount,
 			)
 		},
 		FetchChannelEndpoints: func(chanID uint64) (route.Vertex,