Code quality: reimplement DbChange as variant

[msm-code]

DbChange class is clearly used in ways it was not designed for.

The class definition:

enum class DbChangeType {
    Insert = 1,
    Drop = 2,
    Reload = 3,
    ToggleTaint = 4,
    NewIterator = 5,
    UpdateIterator = 6
};

class DBChange {
   public:
    DbChangeType type;
    std::string obj_name;
    std::string parameter;

    DBChange(const DbChangeType &type, const std::string &obj_name,
             const std::string &parameter = "")
        : type(type), obj_name(obj_name), parameter(parameter) {}
};

Uses parameter (a public field!) as a black-box container for basically everything, and database is expected to dispatch this checking change type.

The most egregious example of this is the UpdateIterator change, which uses string parameter to pass two integers (!).

I think we should rewrite this class using std::variant, similarly to the Command class.

So we'll have use:

using DbChange =
    std::variant<InsertDatasetChange, DropDatasetChange, ......>;

And dispatch using std::visit method.

[msm-code]

That's a nice refactoring, but since the project is, let's say, resource starved, we probably don't have time for that.

It's still something I'd like to see, so if anyone reads this, feel free to reopen if you want to work on this. I probably won't, and this is not critical for anything, so closing this out.