io_queue: Add priority_class_data::on_queued
io_queue: Add priority_class_data::on_queued

This is to be uniform with the rest on_... hooks and
to finally unfriend io_queue.

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

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