PlanManager: use shorter timeout to download items

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.