add async.DelayedJob

[Kamil Samigullin]

job := async.NewDelayedJob(make([]func(), 0, 2))

if cond1 {
  job.Do(...)
}

if cond2 {
  job.Do(...)
}

job.Wait()

[Kamil Samigullin]

motivation: if cond1 is false, then no need to create a goroutine