Bluetooth: Mesh: pb_adv: update delayable work

Switch to the new API. Consolidates reliable sending logic for the first
transmission and the retransmit into one. Adds check for link active in
protocol timeout.

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