[Ch15] Updating code magnets to use executor service

package ch15;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class TestThreads {

  public static void main(String[] args) {

    ThreadOne t1 = new ThreadOne();

    ThreadTwo t2 = new ThreadTwo();

    Thread one = new Thread(t1);

    Thread two = new Thread(t2);

    one.start();

    two.start();

    ExecutorService pool = Executors.newFixedThreadPool(2);

    pool.execute(new ThreadOne());

    pool.execute(new ThreadTwo());

    pool.shutdown();

  }

}

class Accum {

  private static Accum a = new Accum();

  private static final Accum a = new Accum();

  private int counter = 0;

  private Accum() {