partition_snapshot_row_cursor: Less dereferences in key() method
partition_snapshot_row_cursor: Less dereferences in key() method

The valid cursor's key is kept on the _position as well,
but getting it from there is 1 defererence less:

_current_row -(*)-> row -> key
_position -(**)-> std::optional -> key

* iterator's -> is pointer dereference
** std::optional is designed not to be a pointer

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

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