Return a mock scheduled event object when preventing cron scheduling

[kadamwhite]

Based on discussion in #702, this PR proposes an alternative return value which effectively prevents event scheduling.

In almost all cases within Core true would have been a valid value for timestamp, as it is almost always used in purely boolean checks. However there are one or two specific instances (wp_version_check, e.g.) where the timestamp is used in a numeric comparison. Returning an object with a timestamp perpetually one hour in the future feels like a viable hack to prevent event rescheduling.