multi: remove the need for the graphsession package

In this commit, we add a `GraphSession` method to the `ChannelGraph`.
This method provides a caller with access to a `NodeTraverser`. This is
used by pathfinding to create a graph "session" overwhich to perform a
set of queries for a pathfinding attempt. With this refactor, we hide
details such as DB transaction creation and transaction commits from the
caller. So with this, pathfinding does not need to remember to "close
the graph session". With this commit, the `graphsession` package may be
completely removed.

routing/pathfind_test.go

@@ -3221,30 +3221,25 @@ func dbFindPath(graph *graphdb.ChannelGraph,
 		return nil, err
 	}
 
-	graphSessFactory := newMockGraphSessionFactoryFromChanDB(graph)
+	var route []*unifiedEdge
+	err = graph.GraphSession(func(graph graphdb.NodeTraverser) error {
+		route, _, err = findPath(
+			&graphParams{
+				additionalEdges: additionalEdges,
+				bandwidthHints:  bandwidthHints,
+				graph:           graph,
+			},
+			r, cfg, sourceNode.PubKeyBytes, source, target, amt,
+			timePref, finalHtlcExpiry,
+		)
 
-	graphSess, closeGraphSess, err := graphSessFactory.NewGraphSession()
+		return err
+	})
 	if err != nil {
 		return nil, err
 	}
 
-	defer func() {
-		if err := closeGraphSess(); err != nil {
-			log.Errorf("Error closing graph session: %v", err)
-		}
-	}()
-
-	route, _, err := findPath(
-		&graphParams{
-			additionalEdges: additionalEdges,
-			bandwidthHints:  bandwidthHints,
-			graph:           graphSess,
-		},
-		r, cfg, sourceNode.PubKeyBytes, source, target, amt, timePref,
-		finalHtlcExpiry,
-	)
-
-	return route, err
+	return route, nil
 }
 
 // dbFindBlindedPaths calls findBlindedPaths after getting a db transaction from
@@ -3258,8 +3253,7 @@ func dbFindBlindedPaths(graph *graphdb.ChannelGraph,
 	}
 
 	return findBlindedPaths(
-		newMockGraphSessionChanDB(graph), sourceNode.PubKeyBytes,
-		restrictions,
+		graph, sourceNode.PubKeyBytes, restrictions,
 	)
 }