event: ignore event index

Since the content of events is no longer used to drive the Engine
or the Agent, handling all events in order no longer matters.
event: simplify event code

* remove event.EventBus
* move Event definitions into registry pkg
* make EventStream code less async
* establish registry.EventStream interface
registry: watch job namespace for events
Merge pull request #751 from bcwaldon/event-triggers

Simplify event triggers