Try rewriting `pandad` in Python

[adeebshihadeh]

This allows us to remove the parallel C++ Panda interfacing code we maintain just for pandad.

The main challenge here is maintaining the timings and performance of the can and sendcan loops, but it shouldn't be too bad.

As a side benefit, this will also help a bit with boot time, since we currently connect to the pandas twice (pandad.py and pandad.cc).