TaskRunner must ensure Client is initialized for thread before running tasks

If TaskRunner::_runTasks() never runs the body of this loop (for example, due to a shutdown request), then it will never initialize the Client for the thread. Then, when it cancels tasks, it there will be no Client set on the thread. This means it will be illegal for the task to call Client::cc(). TaskRunner::_runTasks() should ensure the Client is initialized before running runSingleTask().