Plane: rangefinder sanity checking

#### Issue details

To support landing locations at a different altitude than launch, the baro.alt - lidar.alt delta sanity check was removed. This allows noisy sensors to control the aircraft with invalid ranges (such as flaring at 50m). Not all sensors are created equal. We need more configuration options to sanity check some sensors more than others. Suggestion: have the sonr.cnt value threshold be a param. Soem sensors need it to be large, such as 30 (3 seconds of continuous good data).
#### Version

3.6
#### Platform

[ ] All
[ ] AntennaTracker
[ ] Copter
[x] Plane
[ ] Rover
#### Airframe type

Fixed wing
#### Hardware type

any
#### Logs

https://drive.google.com/open?id=0B2WywYPDbzkfa3l1ZHkxZ3k5REk (see second land approach flares up at 50m and lidar data is very very lousy on an SF02.