Netty: Add HTTP pipelining support

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.

TODO:
* Check for releasables when adding things to the context
* Refactor NettyHttpChannel to hand over only needed things
* Refactor more for readability
* Far more testing
* Add stats about how many concurrent connection have how many events queued?

Closes #TODOISSUE
Closes #TODOPR