@Override
  public String printUnprocessedTokens() {
    synchronized (unprocessedEventsLock) {
      return printEventIdList(unprocessedTokens.keySet());
    }
  }
Initialize the Executor that handles listener events. Only used by non-primary gateway senders
geode-core/src/main/java/org/apache/geode/internal/cache/wan/serial/SerialGatewaySenderEventProcessor.java