Move advance_to_upper_bound above sstable_mutation_reader
Move advance_to_upper_bound above sstable_mutation_reader

It will be used in sstable_mutation_reader when the reader
will be used to implement sstable::read_row.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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