Waypoints indefinitely re-requested when using MAVProxy with --out option

This is similar to #402, but SITL is not involved.

I run mavproxy in passthrough mode using
```
mavproxy.py --master=/dev/serial/by-id/usb-FTDI_FT230X_Basic_UART_DN02RM7W-if00-port0\
--out=udp:127.0.0.1:14550
```
and let QGroundControl connect to that udp port. The specified serial device is a radio connected to a Cube autopilot running APM:Copter V3.6.11.

Once the parameters are transmitted, I see a new message `re-requesting WPs [0, 1, 2, 3, 4]` every other second in MAVProxy's terminal output.

The workaround of using `wp list` stops the re-requesting of waypoints.

I am using MAVProxy 1.8.15.

Start by reproducing the issue with the shown mavproxy.py command, using a serial master and the --out UDP option, then trace the waypoint request handling. Compare the request behavior with and without --out; done means waypoints are not repeatedly re-requested and the wp list workaround is unnecessary.

Written by the indexing model from the issue text.