when we clear the pvt buffer, we expect it to be empty.

this fixes a bug that can happen if you clear the pvt buffer and
then switch out of pvt mode right away. in that case you may not get
the pvt status message saying the buffer is actually cleared. this
is important for restarting later.
cause the wheel pod to clear the pvt buffers on the servos when leaving
pvt mode.

this should make restarting from pause much easier because it will
cause the motion.cpp code to call startPvtMove at the right time.
Merge remote-tracking branch 'origin/pvt_segment_mode' into pvt_segment_mode