multishard_combining_reader: use optimized optional for the shard reader
multishard_combining_reader: use optimized optional for the shard reader

Use flat_mutation_reader_opt instead of
std::optional<flat_mutation_reader>.

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