Use fixed intervals for detrsp refresh

In !182 (merged) we added a brief wait between writing and reading the detrspmap.
However, the detrspmap writer slowly drifts, since it simply does a sleep(period) after writing. So whatever offset we use, we'd eventually be at risk of a crash.

This change simply checks the time when the script is first launched, and then sleeps the remaining time until the next interval. It doesn't have to be millisecond perfect, it should just prevent any significant drifting day by day.