message: use auto for threadpool init so we can update tbb
diff: #include <gtest/gtest.h>
  TEST(StanUiCommand, threadpool_init) {
  - tbb::task_scheduler_init &scheduler_init = stan::math::init_threadpool_tbb();
  + auto& scheduler_init = stan::math::init_threadpool_tbb();
  EXPECT_TRUE(scheduler_init.is_active());
  scheduler_init.terminate();
  EXPECT_FALSE(scheduler_init.is_active());
  