sstables: Switch is_in_range() to position_in_partition
sstables: Switch is_in_range() to position_in_partition

Makes it immune to #1446 and is a prerequisite for implementing
forwarding in mp_row_consumer.

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