curTimeMicros64() is not a reliable time source for the prepare conflict tracker. 

Motivation: 

PrepareConflictTracker::endPrepareConflict throws an invariant when curTime, computed with curTimeMicros64(), is less than _prepareConflictStartTime (also computed with curTimeMicros64() )

Previously: we saw that curTime, when converted into Unix epoch time, correctly matched the time the invariant message was logged. However, _prepareConflictStart time, converted into Unix epoch time, was a future time that had not occurred yet.