Improve timing behavior

OpENers select loop tends to overshoot, so messages are sent, especially under load, with a one-sided jitter about up to 2 ms. For the standard RPI this is not a problem, but for faster RPIs this will be a problem.
Several steps are needed. First, the elapsed time has to be measured in µs, second the send timers have to be changed to µs granularity. With this changes the timing behavior should be improved.