fix(Core/Grid): Implement missing GridUnload setting (#17569)

* Implement GridUnload setting

* Minor fixes

- Use GetOption instead of deprecated GetBoolDefault.
- Added a missing check for instances in LoadMap
- Replaced some numbers with global defines

* Possible crashfix + minor improvements

- Initialized initialOrientation which I had forgotten (likely cause of crash)
- Readded a previous check in UpdateSplineMovement
- Made i_objectsToRemove and i_worldObjects tos sets as they were previously, instead of unordered_set.

* Update worldserver.conf.dist

* Fix high CPU usage with preload grid enabled.

This should be it.

src/server/game/Maps/Map.h

@@ -768,9 +768,9 @@ private:
     void ProcessRelocationNotifies(uint32 diff);
 
     bool i_scriptLock;
-    std::unordered_set<WorldObject*> i_objectsToRemove;
+    std::set<WorldObject*> i_objectsToRemove;
     std::map<WorldObject*, bool> i_objectsToSwitch;
-    std::unordered_set<WorldObject*> i_worldObjects;
+    std::set<WorldObject*> i_worldObjects;
 
     typedef std::multimap<time_t, ScriptAction> ScriptScheduleMap;
     ScriptScheduleMap m_scriptSchedule;