Refine FieldsReader concept (API Change)

* FieldsReader now requires chunked() and keep_alive()

* serializer logic calls into the reader to calculate
  the message metadata

* Removed some of the previous requirements of FieldsReader

Actions Required:

* Implement chunked() and keep_alive() for user defined FieldsReader types.

test/websocket/stream.cpp

@@ -1004,7 +1004,7 @@ public:
                 }
             };
         // wrong version
-        check(error::handshake_failed,
+        check(http::error::end_of_stream,
             "GET / HTTP/1.0\r\n"
             "Host: localhost:80\r\n"
             "Upgrade: WebSocket\r\n"