Do not reset kvikio thread pool when nthreads is unchanged

[Matt711]

When set_thread_pool_nthreads, we reset the thread pool which drains and detroys the previous one. We avoid that if the number of threads we are setting is unchanged.

See

void defaults::set_thread_pool_nthreads(unsigned int nthreads)
{
  KVIKIO_EXPECT(
    nthreads > 0, "number of threads must be a positive integer", std::invalid_argument);
  thread_pool().reset(nthreads, make_thread_pool_init_task("kvikio"));
}

xref NVIDIA/cudf#23634

[kingcrimsontianyu]

Thanks for the improvement.

[kingcrimsontianyu]

/merge