回滚跨链资产（rollback）

在跨链资产交换的过程中如遇到异常情况，需要读取跨链资产对应数据锁中的PreValue，并根据该值回滚跨链资产交换中涉及资产已发生的变化。可将上述逻辑封装至一个方法中，便于后续在其他智能合约方法（主要是rollbackSend与rollbackRecv）中调用：

/*
 * rollback function will recover the account balance to the previous status
 * @Param account: The name of the account whose balance will be rollback
 * @Param txID: The id of this transaction that transfer units of one account to another account
 */
func rollback(stub shim.ChaincodeStubInterface, txID string, account string) error {
	// get server side's local account lock(with preValue inside)
	accountLockKey := account + lockSuffix
	accountLockBytes, err := stub.GetState(accountLockKey)
	if err != nil {
		return fmt.Errorf("failed to get accountLock state: %v", err)
	}
	if accountLockBytes == nil {
		return nil
	}
	accountLock := AccountLock{}
	err = json.Unmarshal(accountLockBytes, &accountLock)
	if err != nil {
		return fmt.Errorf("failed to unmarshal accountLockBytes: %v", err)
	}
	// recover the balance of server side's local account to the previous balance
	err = stub.PutState(account, []byte(accountLock.PreValue))
	if err != nil {
		return fmt.Errorf("failed to put state of preValue account: %v", err)
	}
	// unlock server side's local account
	err = unlockAccount(stub, txID, account)
	if err != nil {
		return fmt.Errorf("failed to unlock in rollbackRev: %v", err)
	}
	return nil
}