core: wake Waker outside of lock.

Given:

- Broker asleep in poll()
- thread B calling Latch.put()

Previously,

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

Now,

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

mitogen/core.py

@@ -2613,12 +2613,14 @@ class Waker(Protocol):
                             self.stream.transmit_side.fd)
         self._lock.acquire()
         try:
-            if not self._deferred:
-                self._wake()
+            should_wake = not self._deferred
             self._deferred.append((func, args, kwargs))
         finally:
             self._lock.release()
 
+        if should_wake:
+            self._wake()
+
 
 class IoLoggerProtocol(DelimitedProtocol):
     """