Reverse slice queries can skip range tombstones

On disk, we represent range tombstones by a marker at the beginning of the range covered. Since we repeat such markers when they overlap an index block and since an index block is always read in forward order (even in reverse queries), we are guaranteed to see a range tombstone before any column it covers. However, IndexedSliceReader returns the columns of an index block in reverse order and thus can return a range tombstone after columns it covers.

It follows that some range tombstone can be skipped during a reversed range slice. We need to fix IndexedSliceReader to always return range tombstone first (or at least before the first column covered by each range tombstone).