Pipeline friendly request/response

[bjouhier]

The request/response API that ez-streams inherited from streamline-streams does not play very well with the pipeline. Two problems (at least):

• serverResponse is not exactly a writer because encoding is passed in each write call, instead of being configured in the writer. The end method cannot be aligned on the writer API because it is synchronous (so exception handling is out of bounds).
• Applying pipeline operations to a request/response does not yield another request/response. Instead, it yields a reader/writer and all the extra request/response API (headers) is lost. For example, serverRequest.map(someMapper).url does not work.