NewCustomOnce use a custom locker.

NewOnce use sync.Mutex as its locker.

SleepAbout1sTo2s 随机阻塞等待 1 ~ 2s.

CronTab 全局定时器.

GlobalInitMutex 在 init 时被冻结, main 初始化完成后解冻.

Once is an object that will perform exactly one action.