Implement suspend semantics #357

Job	Run time
Publish Image (pytorch-dist-sendrecv-test, examples/pytorch/smoke-dist/Dockerfile) / Publish Image	59s
Publish Image (pytorch-elastic-example-echo, examples/pytorch/elastic/echo/Dockerfile) / Publish Image	6m 45s
Publish Image (tf-dist-mnist-test, examples/tensorflow/dist-mnist/Dockerfile) / Publish Image	35s
Publish Image (pytorch-elastic-example-imagenet, examples/pytorch/elastic/imagenet/Dockerfile) / Publish Image	3m 47s
Publish Image (tf-distributed-worker, examples/tensorflow/distribution_strategy/estimator-API/Doc... / Publish Image	29s
Publish Image (tf-mnist-with-summaries, examples/tensorflow/mnist_with_summaries/Dockerfile) / Publish Image	40s
Publish Image (tf-multi-worker-strategy, examples/tensorflow/distribution_strategy/keras-API/Dock... / Publish Image	3m 48s
Publish Image (tf-smoke, examples/tensorflow/tf_sample/Dockerfile) / Publish Image	38s
	17m 41s

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