rpc: fix support for handlers returning future<no_wait_type>
rpc: fix support for handlers returning future<no_wait_type>

- future<> was misspelt as future<void>
- missing wait_for_reply() overload for handlers returning future<no_wait_type>

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