io_tester: Detach parallel requests submission into a helper
io_tester: Detach parallel requests submission into a helper

The caller will soon have a choice which to call -- this
or a new one.

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

scylladb/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar