flat_mutation_reader: FlatMutationReaderConsumer: support also async consumer
flat_mutation_reader: FlatMutationReaderConsumer: support also async consumer

So that consumer_adapter and other consumers in the future
may return a future from consumer().

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

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