treewide: switch to native reversed format for reverse reads

We define the native reverse format as a reversed mutation fragment
stream that is identical to one that would be emitted by a table with
the same schema but with reversed clustering order. The main difference
to the current format is how range tombstones are handled: instead of
looking at their start or end bound depending on the order, we always
use them as-usual and the reversing reader swaps their bounds to
facilitate this. This allows us to treat reversed streams completely
transparently: just pass along them a reversed schema and all the
reader, compacting and result building code is happily ignorant about
the fact that it is a reversed stream.

query-request.hh

@@ -128,6 +128,13 @@ constexpr auto max_rows_if_set = std::numeric_limits<uint32_t>::max();
 // Specifies subset of rows, columns and cell attributes to be returned in a query.
 // Can be accessed across cores.
 // Schema-dependent.
+//
+// COMPATIBILITY NOTE: the partition-slice for reverse queries has two different
+// format:
+// * legacy format
+// * native format
+// The wire format uses the legacy format. See docs/design-notes/reverse-reads.md
+// for more details on the formats.
 class partition_slice {
     friend class ::partition_slice_builder;
 public: