Rover: add RTK GPS failsafe to handle GPS position jumps when RTK fix is lost

some RTK GPSs (including the Piksi) can suddenly jump the position a few meters when they lose RTK fix. This request from jcantilo in [this discussion](https://discuss.ardupilot.org/t/rtk-fixed-float-jump-in-rover/30444) is to add a failsafe so the vehicle is switched into Hold mode when RTK fix is lost, and then return to the previous mode when RTK fix is restored

Read the linked RTK fixed/float jump discussion, then trace ArduRover's GPS fix handling and vehicle mode transitions to find the relevant failsafe entry points. Done means the rover enters Hold when RTK fix is lost and returns to its prior mode when the fix is restored, with behavior validated against the existing Rover test setup.

Written by the indexing model from the issue text.