routing: limit routing info size during pathfinding

Also the max hop count check can be removed, because the real bound is
the payload size. By moving the check inside the search loop, we now
also backtrack when we hit the limit.

routing/heap.go

@@ -40,6 +40,10 @@ type nodeWithDist struct {
 
 	// nextHop is the edge this route comes from.
 	nextHop *channeldb.ChannelEdgePolicy
+
+	// routingInfoSize is the total size requirement for the payloads field
+	// in the onion packet from this hop towards the final destination.
+	routingInfoSize uint64
 }
 
 // distanceHeap is a min-distance heap that's used within our path finding