Bluetooth: Mesh: Allow sending SNB and PRB in parallel

Private Beacons and Secure Network Beacons are not mutually exclusive by
spec. This means both beacons can be sent simultaneously. For both
beacons spec defines their own observation intervals and related
parameters, which means we need to monitor both beacons types
separately.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>