WorkerPool plugin implementation

The WorkerPool class is one implementation of IWorkerPool in which

• workers from each pool will always run on different nodes. This helps redundancy. It uses the Manager's built-in IDeploymentControl plugin to do this.
• worker-started and worker-died events are passed on to any of its plugins which implement IWorkerLifeCycle, but only when those events come from its own worker types;
• a periodic check ensures the right number of workers are running;
• any IWorkerResourceRequester plugins are used to assess workers' resource needs;
• it tries to reuse old resources which were previously created by this worker pool but which are currently unused, perhaps because the previous netlet terminated unexpectedly.