nharker-core: split up core and nitrite

[tsbonev]

Currently the core domain objects have references to Nitrite due to their indexing. It might be beneficial to split up the domain objects into pure POJOs and have adapted Nitrite objects that hold their index information.
This might aid searching (see #222) as the Nitrite object that is persisted won't clutter the domain with its extra DB-related fields.
Also this will allow deletion to be migrated to a 'deletedOn' field instead of a separate generic table (see #223).