Throttle elasticsearch writes with http agents

For node v0.11.x (and eventually node v0.12.x), the semantics of http.agent have changed, and it looks like the http pool behavior will default to `Infinity` rather than defaulting to `5`.  You probably don't want to make Infinite HTTP requests to your elasticsearch server.  

Testing locally with node v0.11.x, you can see that any number of requests over the `ulimit` (sometimes /4, depending on your OS) will fail to connect.  This will lead to lost data on writes.  

This PR begins an attempt to re-introduce a http agent/throttle to the ES.write transport