multirotor_pos_control: track reference position even if not active to handle reference changes properly
Added vehicle_local_position.ref_surface_timestamp to track surface level changes. Reference altitude updated continuosly when sonar available.
multirotor_pos_control: debug log messages removed
position_estimator_inav: use flow even if it's not accurate if GPS is not available to prevent estimation suspending when no GPS available
Remove vehicle_local_position.ref_surface_timestamp field, don't sync baro_offset and local_pos.ref_alt instead