Use timer_create instead of setitimer

timer_create allows much more high-resolution timers and also
allows controlling the used signal.
Make signal number configurable

For some reason making the signal number configurable significantly
improves performance (by a factor of two), for reasons that elude
me. My previous utilization of SIGRTMIN was incorrect, because
SIGRTMIN is actually a function call and not a constant, but I don't
see how this could have caused a perf regression (rather than just
not working at all).
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