Automatic merge from submit-queue

Delaying deliverer for Federated ReplicaSet

A helper struct to push data to a channel after a given delay. It runs on a single gouroutine and allows updates. An update cancels previous delivery if it was about to happen later than the new one. Otherwise the new update is discarded. 
All data require a string key that is used to identify the data (for updates and de-duplication).

cc: @quinton-hoole @wojtek-t