split Done funcs from workers.Serve / cron.ScheduleTask

[syumai]

What

• Add workers.ServeNonBlock, workers.Done, workers.Ready and cron.Done funcs.

Motivation

• Support HTTP Request and Cron Trigger in Single Worker #130 introduced a non-blocking version of cron.ScheduleTask.
  • A non-blocking version of workers.Serve is also needed, so this PR adds it.
• Additionally, the workers package provides a public Ready() function.
  • It can be used as a common ready signal function.
• The Done() function can be used to wait for a handler or a task to complete.

Usage

func main() {
	// declare handler and task here ...

	// set up the worker
	workers.ServeNonBlock(handler)
	cron.ScheduleTaskNonBlock(task)

	// send a ready signal to the runtime
	workers.Ready()

	// block until the handler or task is done
	select {
	case <-workers.Done():
	case <-cron.Done():
	}
}