Automatic merge from submit-queue

dedup workqueue requeuing

Updates `workqueue.AddAfter` to only perform the add for the earliest requested requeue operation.  An earlier time inserts in the earlier slot and removes the old one.  A later time is ignored.

When using this conjunction with an `AddRateLimited` method, you get charged for the additional retry even though you're only queue once.  

This keeps requeues from multiplying for every add.

@liggitt