map_reduce: allow reducer to take accumulated value by rref
map_reduce: allow reducer to take accumulated value by rref

Message-Id: <c048030723ad42508e90a036efc901a6d7eebc6e@scylladb.com>

syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar