Submit a patch to ES to open MultiSearchResponse and simplify implementation of CoordinateMultiSearchResponse

[rendel]

MultiSearchResponse and MultiSearchResponse#Item have private constructors and variables, making it difficult to extend these two classes. As a work around, we have copy-pasted most of the internal code to replicate their behaviour while being able to extend them. This makes the code more complex and difficult to maintain. The solution would be to propose a patch to the ES community so that MultiSearchResponse becomes extensible by subclassing.