sd-event: instrument sd_event_run() for profiling delays

Set SD_EVENT_PROFILE_DELAYS to activate accounting and periodic logging
of the distribution of delays between sd_event_run() calls.

Time spent in dispatching as well as time spent outside of
sd_event_run() is measured and accounted for.  Every 5 seconds a
logarithmic histogram loop iteration delays since 5 seconds previous is
logged.

This is useful in identifying the frequency and magnitude of latencies
affecting the event loop, which should be kept to a minimum.

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