Add keep alive parameter

[rangoo94]

There is kept connection alive, but actually I don't know what it's a purpose here if this connection is not reused anyway. As first step I added keepAlive parameter which is by default true to keep backward compatibility.

The biggest problem is when you make a lot of requests to ES - you still have all connections from before, so after connections pool is full, "No Living connections" error will be thrown from ElasticSearch.