Special case page handling for DISTINCT queries

Simplify the removal of the first row in a paged range slice page
with DISTINCT if that row was already returned at the end of the
previous page

Patch by Sam Tunnicliffe and Marcus Eriksson; reviewed by Sam Tunnicliffe
and Marcus Eriksson for CASSANDRA-14956

Co-authored-by: Sam Tunnicliffe <sam@beobal.com>
Co-authored-by: Marcus Eriksson <marcuse@apache.org>
4 files changed