partition_snapshot_reader: reverse ck_range when needed by Reversing
partition_snapshot_reader: reverse ck_range when needed by Reversing

Previous commits made it possible to split the responsibility of two
kinds of clustering key ranges in read_next and next_range_tombstone.
Here, the actual reversal takes place and we start passing the actually
reversed ck_range, if Reversing. This reversed ck_range is stored as a
class member, so that the reversal happens just once for each range.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla