code: List<Transaction> getAllTxns() throws IOException {
      Log log = this.persistence.getLog();
      List<Transaction> txns = new ArrayList<Transaction>();
      try(Log.LogIterator iter = log.getIterator(new Zxid(0, 0))) {
        while(iter.hasNext()) {
          txns.add(iter.next());
        }
        return txns;
      }
    }
docs: Returns all the transactions appear in log(It's for testing purpose only).
  
  @return a list of transactions.