-Added ramp-down rate instead of instantly setting ramp to zero when throttle is dropped to bottom. This is to allow "warm-restart" if shutdown was unintentional.
-Actual ESC still goes to zero while throttle is dropped to the bottom, only the ramp counter winds down slowly behind the scenes.
1. it's step towards rearchitecting the current code base ahead of a move to RTOS.
2. internally it uses a MatrixTable for the Quad, Octa, OctaQuad, Y6 frames.
3. it implements the missing stability patch for Octa and OctaQuads (still missing for Y6)
Later check-ins will incorporate into the main ArduCopter code.
