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.

routing/router_test.go

@@ -77,6 +77,7 @@ func (c *testCtx) RestartRouter(t *testing.T) {
 	// With the chainView reset, we'll now re-create the router itself, and
 	// start it.
 	router, err := New(Config{
+		RoutingGraph:       newMockGraphSessionChanDB(c.graph),
 		Graph:              c.graph,
 		Chain:              c.chain,
 		ChainView:          c.chainView,
@@ -140,7 +141,9 @@ func createTestCtxFromGraphInstanceAssumeValid(t *testing.T,
 	sourceNode, err := graphInstance.graph.SourceNode()
 	require.NoError(t, err)
 	sessionSource := &SessionSource{
-		Graph:             graphInstance.graph,
+		GraphSessionFactory: newMockGraphSessionFactoryFromChanDB(
+			graphInstance.graph,
+		),
 		SourceNode:        sourceNode,
 		GetLink:           graphInstance.getLink,
 		PathFindingConfig: pathFindingConfig,
@@ -154,6 +157,7 @@ func createTestCtxFromGraphInstanceAssumeValid(t *testing.T,
 	}
 
 	router, err := New(Config{
+		RoutingGraph:       newMockGraphSessionChanDB(graphInstance.graph),
 		Graph:              graphInstance.graph,
 		Chain:              chain,
 		ChainView:          chainView,
@@ -1763,6 +1767,7 @@ func TestWakeUpOnStaleBranch(t *testing.T) {
 
 	// Create new router with same graph database.
 	router, err := New(Config{
+		RoutingGraph:       newMockGraphSessionChanDB(ctx.graph),
 		Graph:              ctx.graph,
 		Chain:              ctx.chain,
 		ChainView:          ctx.chainView,