transfer: cleanup done+excess handling

- add `SingleRequest->download_done` as indicator that
  all download bytes have been received
- remove `stop_reading` bool from readwrite functions
- move excess body handling into client download writer

Closes #12371

lib/c-hyper.c

@@ -882,6 +882,7 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
      may be parts of the request that is not yet sent, since we can deal with
      the rest of the request in the PERFORM phase. */
   *done = TRUE;
+  Curl_client_cleanup(data);
 
   infof(data, "Time for the Hyper dance");
   memset(h, 0, sizeof(struct hyptransfer));
@@ -892,6 +893,8 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
 
   Curl_http_method(data, conn, &method, &httpreq);
 
+  DEBUGASSERT(data->req.bytecount ==  0);
+
   /* setup the authentication headers */
   {
     char *pq = NULL;