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/Entities/Unit/Unit.cpp

@@ -567,6 +567,13 @@ void Unit::UpdateSplineMovement(uint32 t_diff)
     // this code cant be placed inside EscortMovementGenerator, because we cant delete active MoveGen while it is updated
     SplineHandler handler(this);
     movespline->updateState(t_diff, handler);
+    // Xinef: Spline was cleared by StopMoving, return
+    if (!movespline->Initialized()) {
+        DisableSpline();
+        return;
+    }
+
+    bool arrived = movespline->Finalized();
 
     if (movespline->isCyclic())
     {
@@ -582,8 +589,6 @@ void Unit::UpdateSplineMovement(uint32 t_diff)
         }
     }
 
-    bool arrived = movespline->Finalized();
-
     if (arrived)
     {
         DisableSpline();