* Pet/Guardian AI hook re-organizing:
- Adjust OwnerAttacked/OwnerAttackedBy hooks on CreatureAI to fire for all owned units, not just player pets. This should allow guardians to more reliably recognize valid targets.
- Kill off the AttackedBy hook. While it was defined in CreatureAI.h as virtual, it was only ever invoked for player pets in specific situations. This makes it classic developer bait.
- Adjust PetAI to use DamageTaken instead of AttackedBy.
- Adjust behavior of AttackStart on PetAI to compensate.
(cherry picked from commit 1660bb7d27)