AddTimer adds a timer to the manager.

Cron executes scheduled tasks TODO: if closing Timers'count in single cron call more than timerBacklog will case problem.

NewTimer creates a cron job.

RemoveTimer removes a timer to the manager.

SetTimerBacklog set the timer created/closing channel backlog, A small backlog may cause the logic to be blocked when call NewTimer/NewCountTimer/timer.Stop in main logic gorontine.

LoopForever is a constant indicating that timer should loop forever.

GlobalTicker represents global ticker that all cron job will be executed in globalTicker.

Manager manager for all Timers.

Precision indicates the precision of timer, default is time.Second.