single_key_sstable_reader: move reading code into it's own method
single_key_sstable_reader: move reading code into it's own method

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