code: private void initInflightMessageStore() {
          BTreeIndexFactory<String, StoredPublishEvent> indexFactory = new BTreeIndexFactory<String, StoredPublishEvent>();
          indexFactory.setKeyCodec(StringCodec.INSTANCE);
  
          m_inflightStore = (SortedIndex<String, StoredPublishEvent>) m_multiIndexFactory.openOrCreate("inflight", indexFactory);
      }
docs: Initialize the message store used to handle the temporary storage of QoS 1,2
  messages in flight.