code: Zxid getSnapshotZxid() {
      File snapshot = getSnapshotFile();
      if (snapshot == null) {
        return Zxid.ZXID_NOT_EXIST;
      }
      String fileName = snapshot.getName();
      String strZxid = fileName.substring(fileName.indexOf('.') + 1);
      return Zxid.fromSimpleString(strZxid);
    }
docs: Gets the last zxid of transaction which is guaranteed in snapshot.
  
  @return the last zxid of transaction which is guarantted to be applied.