XnioWorker.shutDownTaskPool() needs 'modifyThread' permission

The exception in the initial description of REM3-183 is caused by XnioWorker.shutDownTaskPool() trying to shut down the thread pool without correct permissions. The ThreadPoolExecutor API docs (http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ThreadPoolExecutor.html) notes:

"SecurityException - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access."

That method should use a doPrivileged() block.