title: Early stopping with tf.estimator, how?
body: <p>I'm using <code>tf.estimator</code> in TensorFlow 1.4 and <code>tf.estimator.train_and_evaluate</code> is great but I need early stopping. What's the prefered way of adding that?</p>
  
  <p>I assume there is some <code>tf.train.SessionRunHook</code> somewhere for this. I saw that there was an old contrib package with a <code>ValidationMonitor</code> that seemed to have early stopping, but it doesn't seem to be around anymore in 1.4. Or will the preferred way in the future be to rely on <code>tf.keras</code> (with which early stopping is really easy) instead of <code>tf.estimator/tf.layers/tf.data</code>, perhaps?</p>