C++ simpale ThreadPool

• support sync call in the same thread. the calling thread need to waits for the worker thread.

  worker.sync_call([]{
   int ans = 0;
   for(int i = 1; i <= 100; i++)
   	ans += i;
   return ans;
  });
  

• support async call in the same thread. the calling thread does not need to wait for the worker thread.

  worker.async_call([]{
   do_something();
  });