io_queue: Move class io_group definition
io_queue: Move class io_group definition

It will need to know the io_queue::config soon.
Just move, no other changes.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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