Core/Maps: Don't use EnsureGridCreated_i() for base map in Map::LoadMap, use EnsureGridCreated() which is thread safe because although the LoadMap() call probably came from a thread safe EnsureGridCreated() call from the current map instance, multiple map instances can load grids for the same base map in the same time, and each of them has it's own mutex, therefore we should rely on the basemap's mutex