Core: Combat/threat system rewrite

Port From (https://github.com/TrinityCore/TrinityCore/commit/34c7810fe507eca1b8b9389630db5d5d26d92e77)

Source/Game/Entities/Pet.cs

@@ -1343,6 +1343,8 @@ namespace Game.Entities
             AddUnitFlag2(UnitFlags2.RegeneratePower);
             SetSheath(SheathState.Melee);
 
+            GetThreatManager().Initialize();
+
             return true;
         }