SSYNC logs tracebacks on unexpected hang-up

This is likely to happen if the client pops a connection timeout, but you can repro it easily enough with curl:

  $ curl -I -X SSYNC http://localhost:6010/sdb1/1
  HTTP/1.1 200 OK
  Content-Type: text/html; charset=UTF-8
  Date: Sat, 01 Aug 2020 00:45:11 GMT
  Transfer-Encoding: chunked

...but then the server logs like

  Aug 1 00:45:11 saio object-6010: 127.0.0.1/sdb1/1 EXCEPTION in ssync.Receiver:
  Traceback (most recent call last):
    File ".../swift/obj/ssync_receiver.py", line 166, in __call__
      for data in self.missing_check():
    File ".../swift/obj/ssync_receiver.py", line 343, in missing_check
      'Looking for :MISSING_CHECK: START got %r' % line[:1024])
  Exception: Looking for :MISSING_CHECK: START got b''
  Aug 1 00:45:11 saio object-6010: STDERR: Traceback (most recent call last):
    File "...s/eventlet/wsgi.py", line 599, in handle_one_response
      write(b'')
    File ".../eventlet/wsgi.py", line 538, in write
      wfile.flush()
    File ".../socket.py", line 604, in write
      return self._sock.send(b)
    File ".../eventlet/greenio/base.py", line 397, in send
      return self._send_loop(self.fd.send, data, flags)
    File ".../eventlet/greenio/base.py", line 384, in _send_loop
      return send_method(data, *args)
  BrokenPipeError: [Errno 32] Broken pipe

That second one's going to be hard to avoid since it's all buried down in eventlet, but we ought to be able to do something for that first one.