Core/Misc: Fix various crashes, also related to multithreading (#19012)

* When iterating groups we have to either do it not in multithreaded context (map updates) or start with checking maps (they are guaranteed to change in single thread update).
* Properly clear ComboPoint references on player remove 
* remove some possible references item may have when it is deleted during save. 
* Also clear all hostile references when unit is removed from map.

src/server/game/Grids/ObjectGridLoader.cpp

@@ -217,7 +217,7 @@ void ObjectGridStoper::Visit(CreatureMapType &m)
     for (CreatureMapType::iterator iter = m.begin(); iter != m.end(); ++iter)
     {
         iter->GetSource()->RemoveAllDynObjects();
-        if (iter->GetSource()->IsInCombat())
+        if (iter->GetSource()->IsInCombat() || !iter->GetSource()->getThreatManager().areThreatListsEmpty())
         {
             iter->GetSource()->CombatStop();
             iter->GetSource()->DeleteThreatList();