engine: collapse Reconciler and dumbReconciler
engine: break out scheduler from reconciler
agent: move AbleToRun to AgentState
agent: verify comment
engine: drop offers; pick first capable agent
engine: refactor clusterState; track changes during reconcile
engine: schedule to agent with least number of units
machine: stop publishing MachineState.LoadedUnits
Merge pull request #728 from bcwaldon/sched

Drop job offering; schedule based on loaded unit count
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