Removing Java8 support

With the end of the GAE Java8 support, we can finally do some code cleanup in the repo:

1/ Remove the RPC way to communicate with Jetty
2/ simplify the existing path from HTTPRequest to Jetty HTTP -> UpRequest translatation ->UpResponse->HTTP
We are duplicating memory there, and we could try to remove entirely UpRequest/UpResponse buffers, assuming we keep the exact same behavior.