Rpc higher priority than build jobs

[aalekseyev]

This PR does two things:

• Refactor Scheduler.Event.Queue.next so we have a tangible list of event sources instead of a large function where the level of indentation roughly determines the priority.

  I think this way it's clearer what's going on. I'm not too worried about allocation of these closures because scheduler events are not supposed to be very numerous.

• Then swap the order of worker_tasks_completed and jobs_completed in this list.

  This is simply to make the priority order make more sense, not really fixing any issue I observed.
  The reason I'm looking into this is that I wanted to use the worker_tasks_completed mechanism for inotify watcher and there we clearly want the same-ish priority as invalidation.