sstables: mutation_reader: Move partition reading code to sstable_data_source
sstables: mutation_reader: Move partition reading code to sstable_data_source

It will be reused for read_row(), which does't create mutation_reader
instance, only sstable_data_source.

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