Netty4 ShuffleHandler: should use 1 boss thread

https://github.com/apache/tez/commit/01847814d32aac6b77a6810ea25d6780fb94b257#diff-5c5dffcaf349fe521937812b49849c59fba3e67fc53d74fb990b7e0ef7ff5e59R182

    bossGroup = new NioEventLoopGroup(maxShuffleThreads, new ThreadFactory() {
      @Override
      public Thread newThread(Runnable r) {
        return new Thread(r, BOSS_THREAD_NAME_PREFIX + bossThreadCounter.incrementAndGet());
      }
    });

With netty3, we initialized NioServerSocketChannelFactory with below constructor:

    selector = new NioServerSocketChannelFactory(
        Executors.newCachedThreadPool(bossFactory),
        Executors.newCachedThreadPool(workerFactory),
        maxShuffleThreads);

this always creates 1 boss and n maxShuffleThreads threads:

    public NioServerSocketChannelFactory(
            Executor bossExecutor, Executor workerExecutor,
            int workerCount) {
        this(bossExecutor, 1, workerExecutor, workerCount);
    }

In Nio model, bossThread take care all bounded socket(listen socket), workerThread take care Accepted-socket (included IO and call event method such as messageReceived).
In case of the boss thread pool, actually the thread pool is unneeded because current implementation acquires only a single thread from it. That thread sleeps on the selector for server socket most of the time, after accepting connection that connection is registered with a worker. From that moment on worker is responsible for serving that connection.