message: make the unique_stream_id a global
diff: namespace minigo {
  namespace {
  +std::atomic<int> unique_stream_id{0};
  +
  uint64_t ChooseSeed(uint64_t seed) {
  if (seed == 0) {
  std::random_device rd;
  @@ -32,8 +34,7 @@ uint64_t ChooseSeed(uint64_t seed) {
  int ChooseStream(int stream) {
  if (stream == 0) {
  - static std::atomic<int> s{0};
  - stream = s++;
  + stream = unique_stream_id.fetch_add(1);
  }
  return stream;
  }
  