Tidy `event_queue` event class requirements. (#202)

The requirements for event types for use in event_queue were restrictive and a bit 'special'.

  * Allow event times of any time which is well ordered by operator>.
  * Allow any event type with a public time member containing the time value.
  * Provide customization point event_time() via ADL for extracting the time value for event types that do not have a time member.
  * Simplify interface: push(begin, end) was only ever used in the unit test; add empty() const method.
  * Add unit test for more flexible functionality.