Split executive into two parts

Not quite ready to be merged yet (```@event_count``` isn't passed through yet for the ackids, and we may need to talk about backwards-compatible config handling), but I'm creating this pull request for some initial review.

I think it's structurally cleaner, and leaves both parts looking less complex in isolation than they were together. Ideally this will make it easier to run event processing and notification generation on different machines.

Fixes #247, or will when it's complete.