partition_snapshot_reader: Be prepared for skipping some row entries
partition_snapshot_reader: Be prepared for skipping some row entries

If some row entries may have to be skipped by the reader then it could
be that _clustering_rows is not empty, but read_next() will return a
disengaged optional because there are no more rows in the current
range. The code assumed that it's never the case, and if read_next()
returns a disengaged optional then we exhousted all ranges. Before
introducing dummy entries this needs to be refactored.

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