future: Define futurize::value_type with future::value_type
future: Define futurize::value_type with future::value_type

It is one less place to change when we stop using std::tuple.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200826040001.792467-5-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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