query::result_view: add get_last_partition_and_clustering_key()
query::result_view: add get_last_partition_and_clustering_key()

Paging needs to get last partition and clustering key (if the latter
exists). Previously, this was done by result_view visitor but that is
suboptimal. Let's add a direct getter for those.

avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla