multi+refactor: let FetchChanInfos not take tx

In preparation for having a clean graph DB interface, refactor
FetchChanInfos so that no transaction can be provided.

channeldb/graph.go

@@ -2374,10 +2374,19 @@ func (c *ChannelGraph) FilterChannelRange(startHeight,
 // skipped and the result will contain only those edges that exist at the time
 // of the query. This can be used to respond to peer queries that are seeking to
 // fill in gaps in their view of the channel graph.
+func (c *ChannelGraph) FetchChanInfos(chanIDs []uint64) ([]ChannelEdge, error) {
+	return c.fetchChanInfos(nil, chanIDs)
+}
+
+// fetchChanInfos returns the set of channel edges that correspond to the passed
+// channel ID's. If an edge is the query is unknown to the database, it will
+// skipped and the result will contain only those edges that exist at the time
+// of the query. This can be used to respond to peer queries that are seeking to
+// fill in gaps in their view of the channel graph.
 //
 // NOTE: An optional transaction may be provided. If none is provided, then a
 // new one will be created.
-func (c *ChannelGraph) FetchChanInfos(tx kvdb.RTx, chanIDs []uint64) (
+func (c *ChannelGraph) fetchChanInfos(tx kvdb.RTx, chanIDs []uint64) (
 	[]ChannelEdge, error) {
 	// TODO(roasbeef): sort cids?
 
@@ -2958,8 +2967,8 @@ func (c *ChannelGraph) isPublic(tx kvdb.RTx, nodePub route.Vertex,
 // key. If the node isn't found in the database, then ErrGraphNodeNotFound is
 // returned. An optional transaction may be provided. If none is provided, then
 // a new one will be created.
-func (c *ChannelGraph) FetchLightningNode(tx kvdb.RTx, nodePub route.Vertex) (
-	*LightningNode, error) {
+func (c *ChannelGraph) FetchLightningNode(tx kvdb.RTx,
+	nodePub route.Vertex) (*LightningNode, error) {
 
 	var node *LightningNode
 	fetch := func(tx kvdb.RTx) error {
@@ -3705,7 +3714,7 @@ func (c *ChannelGraph) markEdgeLiveUnsafe(tx kvdb.RwTx, chanID uint64) error {
 	// We need to add the channel back into our graph cache, otherwise we
 	// won't use it for path finding.
 	if c.graphCache != nil {
-		edgeInfos, err := c.FetchChanInfos(tx, []uint64{chanID})
+		edgeInfos, err := c.fetchChanInfos(tx, []uint64{chanID})
 		if err != nil {
 			return err
 		}

channeldb/graph_test.go

@@ -2685,7 +2685,7 @@ func TestFetchChanInfos(t *testing.T) {
 	// We'll now attempt to query for the range of channel ID's we just
 	// inserted into the database. We should get the exact same set of
 	// edges back.
-	resp, err := graph.FetchChanInfos(nil, edgeQuery)
+	resp, err := graph.FetchChanInfos(edgeQuery)
 	require.NoError(t, err, "unable to fetch chan edges")
 	if len(resp) != len(edges) {
 		t.Fatalf("expected %v edges, instead got %v", len(edges),