Use a bigger buffer for requests/responses

As I'm hoping this will increase the data throughput for responses.

guix-data-service/web/server.scm

@@ -68,6 +68,10 @@
                (request-method request)
                (uri-path (request-uri request)))))
     #:unwind? #t)
+  ;; The fibers web server uses a 1024 byte buffer, so increase this
+  (setvbuf (request-port request)
+           'block
+           (expt 2 16))
   (apply values
          (let-values (((request-components mime-types)
                        (request->path-components-and-mime-type request)))