Bluetooth: Mesh: transport: update delayable work

Switch to the new API. Adds early exits for the ack and retransmit
timers, and replaces a remaining_time() + submit() call with schedule().

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>