library_search with maxHeartbeats 0

Stream: mathlib4

[Floris van Doorn]

@Scott Morrison a new library_search bug report: with set_option maxHeartbeats 0, library_search stops after the first try, presumably because whileAtLeastHeartbeatsPercent and similar functions are not special-casing the 0 maxHeartbeats case. Note that set_option maxHeartbeats 0 is the suggested fix when you get a deterministic timeout.

[Scott Morrison]

!4#3720