Potential REST Connection Leak in InitializationUtils

[jbaiera]

In certain cases, a call to checkIndexExistence may leak a client connection:

https://github.com/elastic/elasticsearch-hadoop/blob/master/mr/src/main/java/org/elasticsearch/hadoop/rest/InitializationUtils.java#L259-L271

This occurs if the following situations are satisfied:

1. The client object passed in is null.
2. es.index.auto.create is set to false (away from the default true value).
3. The write index exists.
4. The client closing method is skipped over.
5. Newly created client object does not close it's resources.

While this is most likely not a problem on most of the integrations, it may pose an issue for long lived tasks.