Fix potential race condition in IndexViewManager during invalidation

There is a potential race condition in the IndexViewManager.invalidate method:

public void invalidate()
{
    View currentView = view.get();

    for (SSTableIndex index : currentView)
    {
        index.markObsolete();
    }

    view.set(new View(context, Collections.emptyList()));
} 

We should getAndSet the view before marking the indexes as obsolete. This would avoid indexes potentially being made obsolete when being accessed.