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/Movement/Spline/Spline.h

@@ -48,7 +48,7 @@ namespace Movement
 
         uint8 m_mode{UninitializedMode};
         bool cyclic{false};
-        float initialOrientation;
+        float initialOrientation{0.f};
 
         enum
         {
@@ -198,7 +198,12 @@ namespace Movement
         }
 
         /** Returns length of the whole spline. */
-        [[nodiscard]] length_type length() const { return lengths[index_hi];}
+        [[nodiscard]] length_type length() const
+        {
+            if (lengths.empty())
+                return 0;
+            return lengths[index_hi];
+        }
         /** Returns length between given nodes. */
         [[nodiscard]] length_type length(index_type first, index_type last) const { return lengths[last] - lengths[first];}
         [[nodiscard]] length_type length(index_type Idx) const { return lengths[Idx];}