compatibility update, port data load code to tf.data, etc.

[wogong]

minor changes to make this repo be compatible with tf 1.10.

1. port data load code to tf.data, https://github.com/tensorflow/tensorflow/blob/r1.4/tensorflow/contrib/data/README.md
2. remove warning, "softmax_cross_entropy_with_logits deprecated", softmax_cross_entropy_with_logits deprecated tensorflow/minigo#37

[Prem95]

For anyone not sure how to make the changes.

1. Use data = tf.data.Dataset instead of tf.contrib.data. Same goes to Iterator
2. Use tf.nn.softmax_cross_entropy_with_logits_v2 instead of tf.nn.softmax_cross_entropy_with_logits

[kratzert]

I'll look over this during the next week. Have some trouble with our newborn at the moment and I don't find much time to cross check the PR. But will come back to this, I promise