routing: pre-allocate the distance map to an estimated node count

Pre-sizing these structures avoids a lot of map resizing, which causes
copies and rehashing of entries. We mostly know that the map won't
exceed that size, and it doesn't affect memory usage in any significant
way.

routing/heap.go

@@ -50,9 +50,10 @@ type distanceHeap struct {
 
 // newDistanceHeap initializes a new distance heap. This is required because
 // we must initialize the pubkeyIndices map for path-finding optimizations.
-func newDistanceHeap() distanceHeap {
+func newDistanceHeap(numNodes int) distanceHeap {
 	distHeap := distanceHeap{
-		pubkeyIndices: make(map[route.Vertex]int),
+		pubkeyIndices: make(map[route.Vertex]int, numNodes),
+		nodes:         make([]nodeWithDist, 0, numNodes),
 	}
 
 	return distHeap