Implement consistency checks

[novoj]

While the transactions are designed - we still do a limited set of consistency checks (for example, mandatory attributes), but we don't check for the existence of referenced entities and so on. We don't do that because we want developers to have control over that, and that requires opening a few knobs here and there. This issue aims to add those checks with the ability to go into an "unsafe" mode where none of those checks are executed if the developer says so.