@Override
  public List<NetworkSend> completedSends() {
    List<NetworkSend> toReturn = sends;
    sends = new ArrayList<NetworkSend>();
    return toReturn;
  }
Returns a list of {@link NetworkSend} sent as part of the last poll.
@return a lit of {@link NetworkSend} initiated previously.
ambry-router/src/test/java/com/github/ambry/router/MockSelector.java