issue #139: eliminate quadratic behaviour from transmit path

Implication: the entire message remains buffered until its last byte is
transmitted. Not wasting time on it, as there are pieces of work like
issue #6 that might invalidate these problems on the transmit path
entirely.

mitogen/core.py

@@ -782,7 +782,7 @@ class Stream(BasicStream):
                 self.on_disconnect(broker)
                 return
             elif written != len(buf):
-                self._output_buf.appendleft(buf[written:])
+                self._output_buf.appendleft(buffer(buf, written))
 
             _vv and IOLOG.debug('%r.on_transmit() -> len %d', self, written)