Return all ContentValues from relation

[sgrekov]

It would be useful to have generated method in _Relation, which will return all content values for particular model, like it made in upsertWithoutTransaction method,
something like
public abstract ContentValues allValues(@nonnull final Model model);

It will be useful when we manually check for local value and depending on result, call insert or update.

[gfx]

Seems good. Will try it in the next version.

[sgrekov]

Thanks! Looking forward to this improvement.