add convert() to future<> futurizer specification
add convert() to future<> futurizer specification

Message-Id: <57f62491313d64b6b63d5a0127fcbbfd64f128b6@scylladb.com>

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