sstables: index_reader: Introduce advance_to_next_partition()
sstables: index_reader: Introduce advance_to_next_partition()

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