Only adjust existing waypoints. Does not add waypoints.

These are defined in PlanManager. Having them here as well causes lost
signaling.

Plus various bug fixes from flight testing

Also remove custom gimbal support

And fixes they found

When downloading mission items from the autopilot we need to retry more
agressively, so use a shorter timeout. If we don't do this, we might not
re-request a mission item in time and the autopilot has already given up
the overall mission transfer.

This fixes the case where a mission request from QGC to autopilot or a
mission item from autopilot to QGC gets lost on the link and needs to be
requested again.