calculate n_threads as "num-cpus +1" instead "num-cpus -1"

This

    /*
      In our threadpool implementation, half the threads are permissive and
      half are strict (when it comes to running lower-priority tasks). So we
      always make sure we have at least two threads, so that there will be at
      least one thread of each kind.
    */
    const int n_threads = get_num_cpus(get_options()) + 1;

could be for multi-cpu systems

    const int n_threads = get_num_cpus(get_options()) - 1;

nowadays ?