Second HTTP overhaul

- Rename message types for clarity
  - HttpMessage -> FullHttpMessage
  - HttpHeader -> HttpMessage
  - HttpRequest -> FullHttpRequest
  - HttpResponse -> FulllHttpResponse
  - HttpRequestHeader -> HttpRequest
  - HttpResponseHeader -> HttpResponse
- HttpContent now extends ByteBufHolder; no more content() method
- Make HttpHeaders abstract, make its header access methods public, and
  add DefaultHttpHeaders
- Header accessor methods in HttpMessage and LastHttpContent are
  replaced with HttpMessage.headers() and
  LastHttpContent.trailingHeaders(). Both methods return HttpHeaders.
- Remove setters wherever possible and remove 'get' prefix
- Instead of calling setContent(), a user can either specify the content
  when constructing a message or write content into the buffer.
  (e.g. m.content().writeBytes(...))
- Overall cleanup & fixes