HTTPServer should use withPipeliningAssistance (and when merged withErrorReporting)

[weissi]

Right now the HTTP pipeline is configured this way:

channel.pipeline.configureHTTPServerPipeline(withServerUpgrade: upgrade)

that sets up an upgrade handler 👍 but doesn't use pipelining assistance (enabled with withPipeliningAssistance: true (available since NIO 1.2.0) not error reporting (enabled with withErrorReporting: true (will be in NIO 1.4.0, PR here)).

• the downside of not using pipelining assistance is that it's pretty hard to work correctly with HTTP pipelining if the withPipeliningAssistance: true isn't used
• the downside of not using automatic error reporting is that if an error like 'headers too large' appears the connection is just closed without sending a response (400 BAD REQUEST) back.

(we can't really enable those options by default because that'd be SemVer breaking ...)