extremely slow param download with multiple vehicles

testing with two vehicles (a ship and a quadplane) on a single RFD900x, the parameter download can take more than 10 minutes. The same link with mavproxy takes only a few seconds. The user tells me that with 6 vehicles param download takes over an hour.
Watching the download, MissionPlanner quickly falls back to single PARAM_VALUE at a time. I found that if during the param download I forwarded the link to mavproxy and then mavproxy did a parameter fetch (not ftp) then MissionPlanner then got the parameters quickly as it saw the parameters fetched by mavproxy.

No source file, test, or entry point is named. Reproduce the parameter download with two vehicles over one RFD900x link, compare Mission Planner with MAVProxy, and trace when Mission Planner falls back to single PARAM_VALUE messages. Done means multi-vehicle parameter downloads complete in a reasonable time without requiring a separate MAVProxy fetch.

Written by the indexing model from the issue text.