future: Split promise::schedule
future: Split promise::schedule

We now have 3 layers:

    void schedule(task* callback) noexcept;
    void forward_state_and_schedule(future_state<T...>* state, task* callback) noexcept;
    void schedule_continuation(continuation_base<T...>* callback) noexcept;

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

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