This PR introduces backward compatibility index tests to test the rolling upgrade process amongst Elasticsearch instances within the same major version. The test executes in three phases. In the first phase, we form a cluster of 2 ES instances on an old version. In the second phase, we keep one of the nodes from the old cluster, kill the other node, but preserve its data directory and start an instance of the current version of ES using the same data directory as the killed instance. In the third phase, we kill the other old version ES instance from the first phase and launch a new instance, using the same data directory as the killed instance. Therefore, during phase 3, we have fully migrated and have all current versions of ES running. In each phase, we run REST tests that index documents and search them, ensuring at each stage that the documents from the previous phase are still there.
Note that because we haven't released a GA yet of 5.0, the tests currently don't start an old version cluster in the first phase. Once GA is released, this will be changed to make the backward compatibility version 5.0, while the current version in the cluster will be 5.x.
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