Fixing body fd access racing condition.

To avoid closing the body fd prematurely, the fd value is moved from
the request struct to the app link.  The body fd should not be closed
immediately after the request is sent to the application due to possible
request rescheduling.

test/test_proxy.py

@@ -196,8 +196,6 @@ Content-Length: 10
         self.assertEqual(resp['body'], payload, 'body')
 
     def test_proxy_parallel(self):
-        self.skip_alerts.append(r'close\(\d+\) failed')
-
         payload = 'X' * 4096 * 257
         buff_size = 4096 * 258