Optimize only once per submap in 2D. (#22)
Optimize only once per submap in 2D. (#22)

The setting was unintentionally set to optimize twice per submap which
harms parallelism.
cartographer-project/cartographer,cartographer-project/cartographer,larics/cartographer,damonkohler/cartographer,damonkohler/cartographer,larics/cartographer,googlecartographer/cartographer,cartographer-project/cartographer,damonkohler/cartographer,larics/cartographer,googlecartographer/cartographer,larics/cartographer,wohe/cartographer,cartographer-project/cartographer,wohe/cartographer,googlecartographer/cartographer,damonkohler/cartographer,wohe/cartographer,wohe/cartographer,googlecartographer/cartographer