The daemon handles multiple nodes, that may or may not be on the same
mesh network. While each node my be seperately configured to beacon or
not beacon, there is nothing gained (except redundent traffic) for each
node to beacon seperately. Beaconing is therefore centralized with the
Network Key the SNB represents, with each *received* beacon delivered
to each node. But for SNBs generated, we keep a count of how many nodes
want beacons sent for a specific key. If 1 or more, we beacon, if 0
nodes want the beacon sent, then we do not beacon.
Since IV Index is used in application nonces, we need to honor IVI flag
not only in network layer crypto, but also in application layer.
This means that if IVI field of incoming packet is different than in
current IV Index, try to decode *both* net and app layers using IV Index
decreased by one.
