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.

graph/db/interfaces.go

@@ -33,7 +33,7 @@ type NodeTraverser interface {
 type Store interface { //nolint:interfacebloat
 	// ForEachNodeDirectedChannel calls the callback for every channel of
 	// the given node.
-	ForEachNodeDirectedChannel(nodePub route.Vertex,
+	ForEachNodeDirectedChannel(v lnwire.GossipVersion, nodePub route.Vertex,
 		cb func(channel *DirectedChannel) error, reset func()) error
 
 	// FetchNodeFeatures returns the features of the given node.