New HttpClientImplV2

This is the V2 version of the HttpClient, relying on in-house libraries only.
2 features are missing:
- http tunnelling
- https
1 caveat: each request timeout has its own file descriptor.
Those limitations will be solved over time.

However, the test coverage for this implementation is far more complet that the curl version. We also benefit of a deeper mastery on the code.

Of course. When it's thread safe!