Now there is a separate SHUTDOWN message that relies only on being
received by the broker thread, the main thread can be hung horribly and
the process will still eventually receive a SIGTERM.
* Support passing Context() objects in function calls and return values.
Now the fakessh demo from the documentation index would work
correctly.
* Since slaves can communicate with each other now, they should also use
the same approach to unpickling as the master already used. Collapse
away all the unpickle extension crap and hard-wire just the 3 types
that support unpickling.
David Wilson