event: Cleanup pass.

- Bashed stale add-remove-client-event ctor and modules.
- Since event unit tests are only one module now, they've been moved up to
the common events dir.
event: Integrated add_remove_client_event_c as an event.

- Fixed forward's name.
- Derived from event_c.
- Added dispatch case to event target.
- Bashed extra cycle_task unit test in event target.