[stream-refactor] BufferedWriter must disconenct Stream, not Protocol

Fix a race where if Stream.on_receive() detects disconnect, it calls
Stream.on_disconnect(), which fires Stream 'disconnect' event, whereas
if BufferedWriter.on_transmit() detects disconnect, it called
Protocol.on_disconnect(), which did not fire the Stream 'disconnect'
event.

Since mitogen.parent listens on Stream's 'disconnect' event to reap
children, this was causing a very difficult to trigger test failure.

Triggered after <1000 runs on a Xeon E5530 with hyperthreading using
hackbench running at the same priority:

    $ hackbench -s 1048576 -l 100000000000 -g 4

mitogen/core.py

@@ -1701,7 +1701,7 @@ class BufferedWriter(object):
             written = self._protocol.stream.transmit_side.write(buf)
             if not written:
                 _v and LOG.debug('%r.on_transmit(): disconnection detected', self)
-                self._protocol.on_disconnect(broker)
+                self._protocol.stream.on_disconnect(broker)
                 return
             elif written != len(buf):
                 self._buf.appendleft(BufferType(buf, written))