Core/Maps: Improvements to terrain swap handling

* Fixed memory leak when unloading grids
* Handle child maps being entered
* Allow chaining more child maps (Draenor -> Tanaan Jungle -> Tanaan Jungle - No Hubs Phase)

src/server/scripts/Commands/cs_misc.cpp

@@ -252,7 +252,7 @@ public:
 
         sDBCManager.Map2ZoneCoordinates(zoneX, zoneY, zoneId);
 
-        Map const* map = object->GetMap();
+        Map* map = object->GetMap();
         float groundZ = object->GetMapHeight(object->GetPositionX(), object->GetPositionY(), MAX_HEIGHT);
         float floorZ = object->GetMapHeight(object->GetPositionX(), object->GetPositionY(), object->GetPositionZ());