Netty: Add HTTP pipelining support

[spinscale]

This adds HTTP pipelining support to netty. Previously pipelining was not
supported due to the asynchronous nature of elasticsearch. The first request
that was returned by Elasticsearch, was returned as first response,
regardless of the correct order.

The solution to this problem is to add a handler to the netty pipeline
that maintains an ordered list and thus orders the responses before
returning them to the client. This means, we will always have some state
on the server side and also requires some memory in order to keep the
responses there.

Pipelining is enabled by default, but can be configured by setting the
http.pipelining property to true|false. In addition the maximum size of
the event queue can be configured.

The initial netty handler is copied from this repo
https://github.com/typesafehub/netty-http-pipelining

Closes #2665

[kimchy]

should be cached thread pool, the default constructor does the right thing here

[spinscale]

yes, also fixed in the HttpPipelineHandlerTest

[kimchy]

few comments, LGTM

[tlrx]

I wonder if it should not also decrement the CountDownLatch if an exception is caught in the SimpleChannelUpstreamHandler ?

Something like

new SimpleChannelUpstreamHandler() {
        @Override
        public void messageReceived(..) {
                ...
                latch.countDown();
        }
        @Override
        public void exceptionCaught(...) {
                latch.countDown();
        }

Just to be sure that the client does not hang indefintily.

[spinscale]

makes sense, will add