Since the Message refactoring from a few weeks back, __iter__ has had
nothing to throw ChannelError if the remote sent _DEAD.

Those fields are always None since the recent fork cleanup work.

14:50:04 E mitogen: mitogen.fork.Stream('fork.7431') crashed
Traceback (most recent call last):
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1287, in _call
    func(self)
  File "/home/dmw/src/mitogen/mitogen/core.py", line 758, in on_receive
    return self.on_disconnect(broker)
  File "/home/dmw/src/mitogen/mitogen/parent.py", line 370, in on_disconnect
    super(Stream, self).on_disconnect(broker)
  File "/home/dmw/src/mitogen/mitogen/core.py", line 721, in on_disconnect
    fire(self, 'disconnect')
  File "/home/dmw/src/mitogen/mitogen/core.py", line 162, in fire
    return [func(*args, **kwargs) for func in signals.get(name, ())]
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1160, in <lambda>
    listen(stream, 'disconnect', lambda: self.on_stream_disconnect(stream))
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1142, in on_stream_disconnect
    for context in self._context_by_id.itervalues():
RuntimeError: dictionary changed size during iteration

Add testimonial from Clojars

-16% reduction in fork cost.

This lets context.shutdown(wait=True) succeed in the master or any
parent.

Central mechanism for deleting all non-Latch file descriptors belonging
to the parent process during fork().

Many dragons present!

Let us override a class attribute to specify prefix from fork.py, rather
than reimplement the same logic.

TLS destructors are not called after fork, therefore we must explicitly
track a global list of free file descriptors, and arrange for that list
to explicitly be destroyed from fork.py.

More hacky layering violations.. force Importer's _context attribute to
our new parent.

This allows context_by_id() in the master to succeed in returning a
Context with a .name matching the context's name, needed for correct
logging.

Previously this would have logged the empty string, because the master
had no mechanism to know the name of a context created by a child.

Mostly just to avoid embarrassing function size, but it may come in
useful for testing later.