@Override
  public List<String> disconnected() {
    if (state == MockSelectorState.IdlePoll) {
      return new ArrayList<>();
    }
    return disconnected;
  }
Returns a list of connection ids destroyed between the last two poll() calls.
@return a list of connection ids.
ambry-network/src/test/java/com/github/ambry/network/SocketNetworkClientTest.java