asyncio: sync with Tulip

* Tulip issue #184: Log subprocess events in debug mode

  - Log stdin, stdout and stderr transports and protocols
  - Log process identifier (pid)
  - Log connection of pipes
  - Log process exit
  - Log Process.communicate() tasks: feed stdin, read stdout and stderr
  - Add __repr__() method to many classes related to subprocesses


* Add BaseSubprocessTransport._pid attribute. Store the pid so it is still
  accessible after the process exited. It's more convinient for debug.

* create_connection(): add the socket in the "connected to" debug log

* Clean up some docstrings and comments. Remove unused unimplemented
  _read_from_self().

Lib/asyncio/streams.py

@@ -15,6 +15,7 @@ from . import events
 from . import futures
 from . import protocols
 from .coroutines import coroutine
+from .log import logger
 
 
 _DEFAULT_LIMIT = 2**16
@@ -153,10 +154,15 @@ class FlowControlMixin(protocols.Protocol):
     def pause_writing(self):
         assert not self._paused
         self._paused = True
+        if self._loop.get_debug():
+            logger.debug("%r pauses writing", self)
 
     def resume_writing(self):
         assert self._paused
         self._paused = False
+        if self._loop.get_debug():
+            logger.debug("%r resumes writing", self)
+
         waiter = self._drain_waiter
         if waiter is not None:
             self._drain_waiter = None
@@ -244,6 +250,12 @@ class StreamWriter:
         self._reader = reader
         self._loop = loop
 
+    def __repr__(self):
+        info = [self.__class__.__name__, 'transport=%r' % self._transport]
+        if self._reader is not None:
+            info.append('reader=%r' % self._reader)
+        return '<%s>' % ' '.join(info)
+
     @property
     def transport(self):
         return self._transport