@Override
  public void onCompletion(Void v, Exception exception) {
    this.exception = exception;
    this.retryState = State.REPLICATION_DONE;
  }
Will be called when the ReplicateBlob operation is done.
@param exception The exception that was reported on execution of the request (if any).
ambry-router/src/main/java/com/github/ambry/router/ReplicateBlobCallback.java