Slice.isEmpty() returns false for some empty slices

Slice.isEmpty is currently defined as comparator.compare(end, start) < 0 but this shouldn't be a strict inequality. Indeed, the way Slice.Bound is defined, having a start equal to an end implies a range like [1, 1), but that range is definitively empty and something we shouldn't let in as that would break merging and other range tombstone related code.

In practice, if you can currently insert such empty range (with something like DELETE FROM t WHERE k = 'foo' AND i >= 1 AND i < 1), and that can trigger assertions in RangeTomstoneList (and possibly other problem).