Implement MPT for neox-2.x

[roman-khimov]

We need to implement MPT for neox-2.x branch starting with neo-project/neo#1560 changes (and any updates of this basic building block). It will also be a separate branch for now, but we need to be prepared for Neo 2 changes and it's also gonna be the base for Neo 3 MPT implementation.

We'll also need to implement key grouping for this to work (or actually move the appropriate code from CLI), the end goal is to be compatible with C# implementation and it mangles keys a little when storing them. I'd suggest keeping that at MPT layer and not changing our actual storage keys (keeping DBs compatible).

It shouldn't be a wrapper around DB (as in C#) but rather a user of Store that DAO can use for some of its methods. And it'd be nice to not have these calculations in dao.Cached because they're not exactly fast and they're not needed for every change (we care mostly about block-level changes at the moment).

[roman-khimov]

See also #851.

[roman-khimov]

Fixed with #990.