[BREAKING] Use event class instead of event for timings. Fixes BUKKIT-4664

TimedRegisteredListener uses a reference to the first event fired. This
causes a memory leak in the server for any references that event has. This
changes TimedRegisteredListener to only store a reference to the class of
the event.

This change is intentionally a breaking change, as it is an obscure part
of the API. A non-breaking change would require the leak to be maintained
or an immediate update for any plugins using the method, as it would be an
indirect break.

A unit test is also included to check behavior of shared superclass
functionality.