This patch adds the new MsgTrack object replacing the
global ack_dict. the ack_dict was not thread safe.
the new MsgTrack is a singleton object that keeps track of
all outbound TextMessage objects. When a TextMessage.send() is called
it is added to the MsgTrack object, and when an ack is received for that
message, the message is removed from the MsgTrack object.
TODO: Add an automatic mechanism for saving the messages in MsgTrack
so that when CTRL-C is called to exit aprsd server, then the MsgTrack
state is saved to storage. When aprsd server is started up again, add
the option to try and reload state of MsgTrack.
This patch also reworked the email thread into an APRSDThread object
that can exit gracefully with CTRL-C.
NOTE: Don't call sleep() with a long time (greater than 5 seconds), as
it causes a delay in exiting aprsd until the last sleep() finishes.
Since aprsd has so many threads now for processing incoming messages and
outgoing messages, we need to coordinate all thread operations so that
they don't block the exiting of the app.
This patch reworks the threading code for processing
messages. This patch also extends the aprslib IS class
to allow us to stop processing the consumer packets when
someone hits CTRL-C correctly. Alloing the app to exit.
