rpc_demo: use the new logger
rpc_demo: use the new logger

... in order to stop using the deprecated, lambda-based
set_logger function.

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