Some implementation notes below.
When iterating in reverse, _last_row is after the current entry
(_next_row) in table schema order, not before like in the forward
mode.
Since there is no dummy row before all entries, reverse iteration must
be now prepared for the fact that advancing _next_row may land not
pointing at any row. The partition_snapshot_row_cursor maintains
continuity() correctly in this case, and positions the cursor before
all rows, so most of the code works unchanged. The only excpetion is
in move_to_next_entry(), which now cannot assume that failure to
advance to an entry means it can end a read.
maybe_drop_last_entry() is not implemented in reverse mode, which may
expose reverse-only workload to the problem of accumulating dummy
entries.
ensure_population_lower_bound() was not updating _last_row after
inserting the entry in latets version. This was not a problem for
forward reads because they do not modify the row in the partition
snapshot represented by _last_row. They only need the row to be there
in the latest version after the call. It's different for reveresed
reads, which change the continuity of the entry represented by
_last_row, hence _last_row needs to have the iterator updated to point
to the entry from the latest version, otherwise we'd set the
continuity of the previous version entry which would corrupt the
continuity.
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