Plane: use new scheduler load_average() function

this also simplifies the main loop
Copter: use the new scheduler load_average() call

this also simplifies the main loop
Rover: use the new load_average() API
Plane: make it easier to find task numbers
Plane: run TECS 50Hz code when throttle suppressed

this fixes auto-launch detection using TECS
Plane: use a larger serial buffer

this makes higher rate MAVLink possible again
Copter: ask for IMU samples at 100Hz

this produces the same result, but makes things simpler in the driver