Atomic update

[vadzimrudkouski]

In the current implementation there is no such thing as update. It could be imitated through combination of Contains, Delete and Put.

            using (ICryptoStorage storage = _factory.Create(Path))
            {
                if (storage.Contains(key))
                {
                    storage.Delete(key);
                }

                storage.AddString(key, data.ToJson());
            }

The problem is that this code is not running inside the transaction. There are possible ways exist when key is successfully deleted but not added and to way to rollback delete functionality.

Atomic update should be added as a part of ICryptoStorage API.