Unify CAN Frame timestamps

The TimeStamp class provides a common means to represent timestamps.

However, it is not consistent between the different plugins. SocketCAN uses the Unix time format, given an absolute time.

The PeakCAN plugin instead uses hardware timestamps and starts counting from zero at least every system restart.

It should be investigated if these timestamps can be unified. The Unix time concept seems appropriate (except that it can not represent times before 1970)