Fixed bottleneck responsible for problem in 4261964849294a52c1e6a2d0aec6f585c06b7050

BeagleBone Black is now able to receive full rate on both CAN buses,
and send it all over Bluetooth (uses *almost* all the CPU).

Changed how messages were put in the buffer,
so that they could be taken out more easily/quickly.
Improved reliability of isoblue systemd service

* Starts up first try now
* Moves buffer file to disk on stop
* stops can-log service from running on same RFCOMM channel