flat_mutation_reader: add optimised flat_mutation_reader_opt
flat_mutation_reader: add optimised flat_mutation_reader_opt

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