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
One problem I just noticed we have here is that we are not checking whether or not we have the full primary key. There could be a case where only some of the fields are passed and the exists() call returns true.