sim: Fix broken event unserialization

sim: Fix broken event unserialization

Events expected to be unserialized using an event-specific
unserializeEvent call. This call was never actually used, which meant
the events relying on it never got unserialized (or scheduled after
unserialization).

Instead of relying on a custom call, we now use the normal
serialization code again. In order to schedule the event correctly,
the parrent object is expected to use the
EventQueue::checkpointReschedule() call. This happens automatically
for events that are serialized using the AutoSerialize mechanism.