core: use an output deque rather than string to improve worst case perf

This probably worsens performance in the common case, but it prevents
runaway producers (see e.g. issue #36) from spending all their CPU
copying around huge strings.

It's also a small step towards a solution to issue #6, which will
replace the output buffer with some sort of fancier queue anyway.

This reduces a particular 40 second run of rsync to 1.5 seconds.

mitogen/core.py

@@ -2,6 +2,7 @@
 import Queue
 import cPickle
 import cStringIO
+import collections
 import errno
 import fcntl
 import imp
@@ -586,13 +587,13 @@ class Stream(BasicStream):
     protocol <stream-protocol>`.
     """
     _input_buf = ''
-    _output_buf = ''
 
     def __init__(self, router, remote_id, **kwargs):
         self._router = router
         self.remote_id = remote_id
         self.name = 'default'
         self.construct(**kwargs)
+        self._output_buf = collections.deque()
 
     def construct(self):
         pass
@@ -643,14 +644,19 @@ class Stream(BasicStream):
     def on_transmit(self, broker):
         """Transmit buffered messages."""
         IOLOG.debug('%r.on_transmit()', self)
-        written = self.transmit_side.write(self._output_buf)
-        if written is None:
-            LOG.debug('%r.on_transmit(): disconnection detected', self)
-            self.on_disconnect(broker)
-            return
 
-        IOLOG.debug('%r.on_transmit() -> len %d', self, written)
-        self._output_buf = self._output_buf[written:]
+        if self._output_buf:
+            buf = self._output_buf.popleft()
+            written = self.transmit_side.write(buf)
+            if not written:
+                LOG.debug('%r.on_transmit(): disconnection detected', self)
+                self.on_disconnect(broker)
+                return
+            elif written != len(buf):
+                self._output_buf.appendleft(buf[written:])
+
+            IOLOG.debug('%r.on_transmit() -> len %d', self, written)
+
         if not self._output_buf:
             broker.stop_transmit(self)
 
@@ -659,7 +665,7 @@ class Stream(BasicStream):
         pkt = struct.pack('>hhLLL', msg.dst_id, msg.src_id,
                           msg.handle, msg.reply_to or 0, len(msg.data)
         ) + msg.data
-        self._output_buf += pkt
+        self._output_buf.append(pkt)
         self._router.broker.start_transmit(self)
 
     def send(self, msg):