partition_snapshot_row_cursor: construct the clustering_row directly in row()
partition_snapshot_row_cursor: construct the clustering_row directly in row()

Currently row() creates an empty clustering_row, then applies deletable_rows
from the cursor to the empty clustering_row.
But the apply logic is unnecessary for the first apply(), and it's cheaper
to simply copy the row.

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