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_test.go

@@ -17,7 +17,7 @@ func TestHeapOrdering(t *testing.T) {
 
 	// First, create a blank heap, we'll use this to push on randomly
 	// generated items.
-	nodeHeap := newDistanceHeap()
+	nodeHeap := newDistanceHeap(0)
 
 	prand.Seed(1)