Adds support for extended advertiser commands in the mesh. This doubles
throughput for common packet sending, and significantly improves timing
accuracy for the Friend and Low Power features.
The proxy module's advertisement control has been moved inside the adv
module to abstract away the different advertiser modes.
The extended advertiser mode does not need an advertising thread to
operate, and ends up with a net reduction in RAM usage.
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>
