Update examples to TF 1.11 APIs

[leewyang]

First pass at updating examples for TF 1.11:

• Removed examples/slim since TF-Slim appears to be deprecated in favor of tf.keras and tf.layers.
• Replaced default examples/mnist/*/mnist_{spark|dist}.py files with _dataset variants (and updated to tf.train.MonitoredTrainingSession).
• Updated examples/mnist/keras.

Also:

• Avoid call to terminate feed (if already terminating).