Expose task queue size for ExecutorService runners.

The LegacyScheduler class creates an ExecutorService object to run tasks. This FixedThreadPool has a task queue that contains the list of pending tasks to execute. We should expose the number of items on its queue for Pollerd and Collectd so users can get an idea of how much pending work is there for these services.

    public LegacyScheduler(final String parent, final int maxSize) {
        m_status = START_PENDING;
        m_runner = Executors.newFixedThreadPool(maxSize, new LogPreservingThreadFactory(parent, maxSize));
        m_queues = new ConcurrentSkipListMap<Long, BlockingQueue<ReadyRunnable>>();
        m_scheduled = 0;
        m_worker = null;
    }

Having the task queue size would be important in the case when ActiveThreads == PeekPoolThreads == MaxPoolThreads.