mutation_reader: filtering_reader: fill_buffer: futurize inner loop
mutation_reader: filtering_reader: fill_buffer: futurize inner loop

Prepare for futurizing next_partition().

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla