io_queue: Shuffle dispatching paths
io_queue: Shuffle dispatching paths

The desc object will do all the per-class accounting related
to requests completion. It's better to keep _all_ the accounting
in desc. This will let us remove the priority class reference
from the queued request object and will simplify the io_queue
API.

Start this change with movind the on-dispatch accounting from
queued request into desc.

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

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