graph/db: version ForEachNodeDirectedChannel and cacheable traversal

Add a gossip version parameter to ForEachNodeDirectedChannel on the
Store interface and both DB implementations (KVStore, SQLStore). The
NodeTraverser and routing.Graph interfaces remain unversioned since
pathfinding operates on the merged cross-version cache view.

The cache population in populateCache is updated in the same commit
because it is logically atomic with the versioning changes: the graph
cache is the unversioned, merged view used by pathfinding, so it must
be populated with data from all gossip versions. Without this change,
only v1 data would be loaded into the cache, making v2 nodes and
channels invisible to pathfinding.

routing/pathfind.go

@@ -1461,8 +1461,7 @@ func processNodeForBlindedPath(g Graph, node route.Vertex,
 	// Now, iterate over the node's channels in search for paths to this
 	// node that can be used for blinded paths
 	err = g.ForEachNodeDirectedChannel(
-		node,
-		func(channel *graphdb.DirectedChannel) error {
+		node, func(channel *graphdb.DirectedChannel) error {
 			// Keep track of how many incoming channels this node
 			// has. We only use a node as an introduction node if it
 			// has channels other than the one that lead us to it.