graph/db: init KVStore outside of ChannelGraph

So that we can pass in the abstract V1Store in preparation for adding a
SQL implementation of the KVStore.

routing/pathfind_test.go

@@ -166,9 +166,11 @@ func makeTestGraph(t *testing.T, useCache bool) (*graphdb.ChannelGraph,
 
 	t.Cleanup(backendCleanup)
 
+	graphStore, err := graphdb.NewKVStore(backend)
+	require.NoError(t, err)
+
 	graph, err := graphdb.NewChannelGraph(
-		&graphdb.Config{KVDB: backend},
-		graphdb.WithUseGraphCache(useCache),
+		graphStore, graphdb.WithUseGraphCache(useCache),
 	)
 	if err != nil {
 		return nil, nil, err