mutable managed executor services for thread pool services

Currently thread pool services in the threads subsystem implement Service<ExecutorService>. To improve usability, it is suggested to change them like

BoundedQueueThreadPoolService implements Service<ExecutorService>

becomes BoundedQueueThreadPoolService implements Service<ManagedQueueExecutorService>

public class ManagedQueueExecutorService implements ExecutorService

private final QueueExecutor toManage
private final BlockingExecutorService delegate;

// Package protected for subsys write-attribute handlers
void setCoreThreads(int coreThreads)

{ toManage.setCoreThreads(coreThreads); }

... other setters, plus delegate the ExecutorService calls
}