graph/db: refactor source node fetching logic

Small refactor to introduce a re-usable getSourceNode function that we
can then make use of in the upcoming kvdb->SQL migration logic.

graph/db/sql_store.go

@@ -3699,7 +3699,7 @@ type srcNodeInfo struct {
 	pub route.Vertex
 }
 
-// getSourceNode returns the DB node ID and pub key of the source node for the
+// sourceNode returns the DB node ID and pub key of the source node for the
 // specified protocol version.
 func (s *SQLStore) getSourceNode(ctx context.Context, db SQLQueries,
 	version ProtocolVersion) (int64, route.Vertex, error) {