map_reduce: reducer_traits: templatize the reducer smart ptr
map_reduce: reducer_traits: templatize the reducer smart ptr

So we can define a different one in the next patch.

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

scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar