Release locks in ArrayHandleVirtual control portals

This fixes an issue where getting a ReadPortal or a WritePortal from an ArrayHandleVirtual could cause a deadlock from a held token even if the returned portal was detached or destroyed.

The problem was that ArrayHandleVirtual was keeping a reference to the ArrayPortal from the concrete array. This was because the returned ArrayPortalRef, which was designed to work on both control and execution environments, had no good way to destroy the portal. This meant that the ArrayHandleVirtual was caching a copy of the concrete array's portal. This was not a great idea before because the array could get invalidated. It is worse now because it keeps the concrete array locked.

Fixed the problem by subclassing vtkm::ArrayPortalRef to make a control-specific version that will delete the concrete portal on its own destruction.