使用方法参考：
#include "ThreadPool.h"
#include <iostream>
using namespace std;
int main()
{
ThreadPool pool;
pool.start(4);
for (size_t i = 0; i < 1024*1024; i++)
{
pool.addTask([=]()
{
cout << "i value is : " << i << endl;
});
}
return 0;
}