Responses from multiple Waypoint downloads may not be replicated in outs

The following integration test (which also uses dronekit-python 2.9.0) demonstrates a failed case. Where repeated waypoint downloads succeed on a direct telemetry link to APM SITL 3.3. But failed when the link is brokered by MAVProxy 1.5.7:

https://gist.github.com/tribbloid/214491f50581bb5d17f2c419267304f1

It is unknown whether the download is blocked by MAVProxy, or if it is ignored by dronekit-python. Please resolve it if it is not caused by MAVProxy

Start with the integration test in the linked gist and reproduce repeated waypoint downloads against APM SITL 3.3 directly and through MAVProxy 1.5.7, using dronekit-python 2.9.0. Trace whether MAVProxy blocks or fails to replicate the responses, or whether dronekit-python ignores them; done means the responsible component and a reproducible result are established.

Written by the indexing model from the issue text.