"got MISSION_REQUEST: use MISSION_REQUEST_INT" message soon after startup

I think that @peterbarker may have added this warning to ArduPilot recently to help us uncover any ground stations that are still using MISSION_REQUEST instead of MISSION_REQUEST_INT.

This can be recreated in SITL by running master Copter and connecting with MP soon after startup.

Reproduce the report in SITL using master Copter and connect with Mission Planner soon after startup. Trace how Mission Planner handles MISSION_REQUEST and MISSION_REQUEST_INT, then verify the startup interaction no longer produces the reported message and that mission requests still work.

Written by the indexing model from the issue text.