worker id overflow

RoutingWorker* RoutingWorker::pick_worker()

{

    static int id_generator = 0;

    int id = this_unit.id_min_worker

        + (mxb::atomic::add(&id_generator, 1, mxb::atomic::RELAXED) % this_unit.nWorkers);

    return get(id);

}

In code above, id_generator is int, when it overflows, it becomes negative integer, then we will have an invalid worker_id in 2.2.

in 2.3, it's different, but `get(id)` will return nullptr, this requires the calling method to check if worker is empty. Apparently, gw_process_one_new_client is not doing it:

1487     mxs::RoutingWorker* worker = mxs::RoutingWorker::pick_worker();

1488

1489     worker->execute([=]() {

suggested fix:

change id_generator to unsigned type