this adds a switch debouncer, similar to the one used in
ArduCopter. I'm adding this after a flight on the weekend where noise
on the control mode channel caused a mode change away from auto.
To prevent this change adding excessive mode switch latency, it also
moves the reading of the control switch to the 10Hz loop, away from
the 3.3Hz loop. That gives us 0.2s delay in mode switch changes and
allows for spikes in the control mode for 0.1 seconds without changing
mode.
This is useful for switching back to the mode indicated by the control
switch after a failsafe or fence breach. I was just changing modes
each time, but it is far more elegant to have a spring loaded switch
for "I am ready to take control again", rather than continually
changing modes.
