CoordinatedWorker enhancements

[sclasen]

CoordinatedWorkers usually need to be stateful, so the handler code should create one per task, rather than just one.

When a Coordinated worker starts up, there should be a callback that allows it to resume heartbeating any existing tasks.

func ResumeTasks() map[swf.ActivityTask]some.WorkerType{

}

which get massaged into their tasks being heartbeat started, and Tick loop being called.