Overhaul mode A/C matching to be much cheaper.

The old matching process which tracked mode A values as pseudo-aircraft
got very, very expensive with a large number of mode A/C messages (and
with lots of single-bit errors, which seems common with a Beast doing
the reception)

Instead just count A/C messages directly into a 4096-entry array (which
is very fast) and periodically scan the mode S aircraft list to see if
we can match anything up (which is fixed overhead + cost proportional
to the number of mode S aircraft)

mode_s.c

@@ -1806,7 +1806,7 @@ void useModesMessage(struct modesMessage *mm) {
             // If this is the second message, and we
             // squelched the first message, then re-emit the
             // first message now.
-            if (!Modes.net_verbatim && a->messages == 2) {
+            if (!Modes.net_verbatim && a && a->messages == 2) {
                 modesQueueOutput(&a->first_message, a);
             }
             modesQueueOutput(mm, a);