@Override
  public List<String> connected() {
    List<String> toReturn = connected;
    connected = new ArrayList<String>();
    return toReturn;
  }
Returns a list of connection ids created between the last two poll() calls (or since instantiation if only one
{@link #poll(long, List)} was done).
@return a list of connection ids.
ambry-router/src/test/java/com/github/ambry/router/MockSelector.java