Make Entity::isNew() only support true/false

Having an indeterminate state complicates life. Having only true/false makes interacting with the ORM a bit simpler. Table::save() will still determine if an entity is truely new when saving and the primary key is
present. Delete operations assume you know what you are doing, and don't try to help as destroying data is generally more dangerous/difficult to recover vs. having too much data.

Refs #4050
Actually the primary keys are checked on line 1249 just before the insert is performed. Update case also checks keys.