code: public ProcessTxnResult processTxn(TxnHeader hdr, Record txn) {
          return dataTree.processTxn(hdr, txn);
      }
docs: the process txn on the data
  @param hdr the txnheader for the txn
  @param txn the transaction that needs to be processed
  @return the result of processing the transaction on this
  datatree/zkdatabase