issue #456: loosen Waker.defer() shutdown test a little

Allow messages to continue being queued during the shutdown period,
right up until the final loop iteration, even though this is racy, as
too many things depend on .defer() during exit right now.

This doesn't hurt the spirit of the check: it still catches the worst
situation where $user accidentally shut down Broker then tried to
continue using it.

mitogen/core.py

@@ -2122,8 +2122,8 @@ class Waker(BasicStream):
 
     broker_shutdown_msg = (
         "An attempt was made to enqueue a message with a Broker that has "
-        "already begun shutting down. It is likely your program called "
-        "Broker.shutdown() too early."
+        "already exitted. It is likely your program called Broker.shutdown() "
+        "too early."
     )
 
     def defer(self, func, *args, **kwargs):
@@ -2138,7 +2138,7 @@ class Waker(BasicStream):
         if threading.currentThread().ident == self.broker_ident:
             _vv and IOLOG.debug('%r.defer() [immediate]', self)
             return func(*args, **kwargs)
-        if not self._broker._alive:
+        if self._broker._exitted:
             raise Error(self.broker_shutdown_msg)
 
         _vv and IOLOG.debug('%r.defer() [fd=%r]', self, self.transmit_side.fd)
@@ -2564,6 +2564,7 @@ class Broker(object):
 
     def __init__(self, poller_class=None):
         self._alive = True
+        self._exitted = False
         self._waker = Waker(self)
         #: Arrange for `func(\*args, \**kwargs)` to be executed on the broker
         #: thread, or immediately if the current thread is the broker thread.
@@ -2724,6 +2725,7 @@ class Broker(object):
         except Exception:
             LOG.exception('_broker_main() crashed')
 
+        self._exitted = True
         self._broker_exit()
         fire(self, 'exit')