@Nullable
  private SerialSecondaryGatewayListener getAndInitializeCacheListener() {
    if (!sender.isPrimary()) {
      final SerialSecondaryGatewayListener listener = new SerialSecondaryGatewayListener(this);
      initializeListenerExecutor();
      return listener;
    }

    return null;
  }
When the Number of unchecked events exceeds this threshold and the number of tokens in the map
exceeds this threshold then a check will be done for old tokens.
geode-core/src/main/java/org/apache/geode/internal/cache/wan/serial/SerialGatewaySenderEventProcessor.java