core: wake Latch outside of lock.

Given:

- thread A asleep in Latch._get_sleep()
- thread B calling Latch.put()

Previously,

- B takes lock,
- B wakes socket by dropping GIL and writing to it
- A wakes from poll(), acquires GIL only to find Latch._lock is held
- A drops GIL, sleeps on futex() for _lock
- B wakes, acquires GIL, releases _lock
- A wakes from futex(), acquires lock

Now,

- B takes lock, updates state, releases lock
- B wakes socket by droppping GIL and writing to it
- A wakes from poll(), acquires GIL and _lock
- Everyone lives happily ever after.

mitogen/core.py

@@ -2489,17 +2489,20 @@ class Latch(object):
                 raise LatchError()
             self._queue.append(obj)
 
+            wsock = None
             if self._waking < len(self._sleeping):
                 wsock, cookie = self._sleeping[self._waking]
                 self._waking += 1
                 _vv and IOLOG.debug('%r.put() -> waking wfd=%r',
                                     self, wsock.fileno())
-                self._wake(wsock, cookie)
             elif self.notify:
                 self.notify(self)
         finally:
             self._lock.release()
 
+        if wsock:
+            self._wake(wsock, cookie)
+
     def _wake(self, wsock, cookie):
         written, disconnected = io_op(os.write, wsock.fileno(), cookie)
         assert written == len(cookie) and not disconnected