core: Modernize `lowres_clock_impl` declaration
core: Modernize `lowres_clock_impl` declaration

- Prefer `using` to `typedef`.

- Prefer explicit default initialization.

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