mutation_reader: mutation_fragment_merger: close underlying producer
mutation_reader: mutation_fragment_merger: close underlying producer

This will be needed by the merging_reader.

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

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