code: Zxid getZxidFromFileName(File file) {
      String fileName = file.getName();
      String strZxid = fileName.substring(fileName.indexOf('.') + 1);
      return Zxid.fromSimpleString(strZxid);
    }
docs: Given the log file, finds out the smallest allowed zxid for thi file. It's
  infered by looking at the name of the file.
  
  @return the smallest allowed zxid in this log file.