Support a simple QueuedExecutor that can be drained (in the calling thread or a separate executor)

[sanjaypujare]

While creating tests I thought it would be nice to have what I call a QueuedExecutor that can be supplied to code-under-test and then I can verify the tasks that are queued and optionally execute them in my thread or a separate executor. This would look something like:

  class QueuedExecutor implements Executor {
    /** A list of Runnables to be run in order. */
    private final Queue<Runnable> runQueue = new ConcurrentLinkedQueue<>();

    @Override
    public synchronized void execute(Runnable r) {
      runQueue.add(checkNotNull(r, "'r' must not be null."));
    }

    public void drain() {
        drain(MoreExecutors.directExecutor());
    }

    public void drain(Executor executor) {
      executor.execute(new Run() {
        Runnable r;
        synchronized(runQueue) {
          while ((r = runQueue.poll()) != null) {
            try {
              r.run();
            } catch (RuntimeException e) {
              // Log it and keep going.
              //...
            }
          }
        }
      });
    }
  }

And MoreExecutors can have a method to return one e.g. newQueuedExecutor Note that this is different from the behavior of MoreExecutors.newSequentialExecutor. I can create a PR to add this depending on feedback.