this patch adds the new PacketCollector class.
It's a single point for collecting information about
packets sent and recieved from the APRS client.
Basically instead of having the packetlist call the seen list
when we get a packet, we simply call the PacketCollector.rx(),
which in turn calls each registered PacketMonitor class.
This allows us to decouple the packet stats like classses inside
of APRSD. More importantly, it allows extensions to append their
own PacketMonitor class to the chain without modifying ARPSD.
