autopilot: move determining chanSize from heuristic to agent

Since we want to combine scores from multiple heuristics, things get
complicated if the heuristics report their own individual channel sizes.
Therefore we change the NodeScores interface slightly, letting the agent
specify the wanted channel size, and let the heuristic score the nodes
accordingly.

autopilot/agent.go

@@ -547,10 +547,16 @@ func (a *Agent) openChans(availableFunds btcutil.Amount, numChans uint32,
 		return fmt.Errorf("unable to get graph nodes: %v", err)
 	}
 
+	// As channel size we'll use the maximum channel size available.
+	chanSize := a.cfg.Constraints.MaxChanSize()
+	if availableFunds-chanSize < 0 {
+		chanSize = availableFunds
+	}
+
 	// Use the heuristic to calculate a score for each node in the
 	// graph.
 	scores, err := a.cfg.Heuristic.NodeScores(
-		a.cfg.Graph, totalChans, availableFunds, nodes,
+		a.cfg.Graph, totalChans, chanSize, nodes,
 	)
 	if err != nil {
 		return fmt.Errorf("unable to calculate node scores : %v", err)