SSE Support

Hi all,
As my company needs stress tests for SSE, we decided to start the implementation of the SSE support within Gatling. This pull request is therefore for #1734.

Note that:
* this is a early submission for review and feedback
* the implementation is inspired of what has been done for web-socket support
* the pre-requisite for the ahc client is to set the request timeout to -1 (we didn't want to touch the ahc client)
* right now, the retry is not (yet) implemented
* the checks supported are those of Ws (as we didn't want to duplicate code)

A sample code with a SSE server can be found there: https://github.com/ctranxuan/gatling2-sse-tests

Any feedbacks would be greatly appreciated. Meanwhile, we are starting to use this implementation to perform our own stress tests, so we might update the code to fix any issues that would have been found.

Best regards,

C.
Why not build a Map and use `Jackson.toJsonString`?