race condition on gdal_grid::DoCloseStorage from

[MaartenHilferink]

scenario, see #829:

• ReadUnitRange from TIFF -> DoCloseStorage() -> if (m_IsOpen) DoCloseStorage() -> m_HDc = nullptr; m_IsOpen= false;
• read ReadData, and in another thread of the same TIFF -> DoOpenStorage ? StorageMetaInfo::DoOpen -> if (!m_IsOpen) DoOpen()
  Consider similar scenario's with other StorageManagers()

Possible solution, use m_HDC as reusable shared resource; use m_StorageMutex to avoid race conditions on reading and/or changing this and/or m_IsOpen.