Use elasticsearch's routing feature

Made some change in order to use elasticsearch's routing feature.  We use the stream_id as routing_key , but we could maybe use something else. The mapping shoud be modified to use
the routing key, i changed it manually but could be done by graylog2...

{

    "_default_": { "_routing": {

            "required": true,

            "path": "streams"

        }

    }

}