io_queue: Add io_direction_and_length::read/write aliases
io_queue: Add io_direction_and_length::read/write aliases

Less typing in io_queue.cc now and further

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

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