read_callback: move to SessionHandle from connectdata

With many easy handles using the same connection for multiplexing, it is
important we store and keep the transfer-oriented stuff in the
SessionHandle so that callbacks and callback data work fine even when
many easy handles share the same physical connection.

lib/multi.c

@@ -1513,7 +1513,7 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
                    __LINE__));
 
       /* read/write data if it is ready to do so */
-      result = Curl_readwrite(data->easy_conn, &done);
+      result = Curl_readwrite(data->easy_conn, data, &done);
 
       k = &data->req;