repair: Allow default construct for repair_row
repair: Allow default construct for repair_row

All members of repair_row are now optional. Enable the default
constructor so that _row_buf.resize() can work.

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