partition_snapshot_row_cursor: Relax vector of iterators
partition_snapshot_row_cursor: Relax vector of iterators

The cursor maintains a vector of iterators that correspond to
each of the versions scanned. However, only the iterator in
the latest one is really needed, so the whole vector can be
reduced down to an optional.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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