Allows to get event time for a head item in the queue. The usecase
if waiting for the next event *OR* I/O completion. I/O completion may
happen before event triggers, and then wait should continue for the
remaining event time (or I/O completion may schedule another earlier
event altogether).
The new function has a strongly provisional status - it may be converted
to e.g. peek() function returning all of the event fields, not just time.
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