Get rid of mock_database.py and cleaner structure

Mainly:

• contextual.py has nothing to do with persistent storage/ package, moved out to core/
• We had a duplicated clive/core/ and clive/__private/core. Since we don't have a public interface (yet?), better to place everything under __private/core
• same as above for clive/models
• remove mock_database.py in favor of storing models defined there in more appropriate places (UpdateNodeDataCommand related)