Bug finally fixed

Since I didn't have much time it took me a while to fix a bug when doing transaction rollback. I've known that this bug exists for some time but I just couldn't find the cause because the rollback just created the state because from which it failed some time later. Just today, I finally found where the problem was. I actually found a couple of problems (some potential) which are now fixed.

Now I need to perform a big high-load test which will simulate all forms of usages, including crashing the process running the storage to see if transaction system will correctly rollback the transaction.