Fixed issue with packet object and prepare()

If any part of the code had a packet object and called prepare()
on it, it would create a msgNo if it wasn't set.  Sometimes we
get packets on the wire/RF that don't have a msgNo, so adding one
locally is wrong.   We should ONLY create a msgNo on a packet that
is destined for transmission.

This patch ensures that even if prepare() is called, that only
packets that are created locally for TX get a msgNo unless it
already exists from RX'd packets.

tests/client/test_aprsis.py

@@ -61,7 +61,7 @@ class TestAPRSISClient(unittest.TestCase):
                     "connected": True,
                     "filter": "m/50",
                     "login_status": {"message": mock.ANY, "success": True},
-                    "server_keepalive": mock_client.aprsd_keepalive,
+                    "connection_keepalive": mock_client.aprsd_keepalive,
                     "server_string": mock_client.server_string,
                     "transport": "aprsis",
                 },