This patch adds the new PacketFilter class as a generic mechanism
for doing packet filtering during the packet processing phase of
recieving packets.
The packet phases are:
1. reception and stats collection
2. packet processing.
Each phase has a single thread for handling that phase.
Phase 1:
The ARPSDRXThread connects to the APRS client, and gets packets
from the client. Then it puts the packet through the Collector
for stats and tracking. Then the packet is put into the packet_queue.
Phase 2:
Packets are pulled from the packet_queue. Then packets are run
through the PacketFilter mechanism, then processed depending
on the command being run.
By default there is 1 loaded packet filter, which is the
DupePacketFilter which removes "duplicate" packets that aprsd has
already seen and processed within the configured time frame.
This PacketFilter mechanism allows an external extension or plugin
to add/remove packet filters at will depending on the function
of the extension or plugin. For example, this allows an extension
to get a packet and push the packet into an MQTT queue.
This adds a try block around the aprslib.parse() for packets incoming
on the KISS interface. Often times we'll get invalid packets and
this prevents stack dumps to the log.
This new collector allows external extensions and plugins to
get callbacks during the keepalive thread runs. Every 60 seconds
the keepalive thread will do some checks and log some entries in the
logfile to keep the user/admin aware of the state of aprsd itself.
