TODO:

  - contexts!

  - notifications!

  - @expose decorator

  - when shutting down a dispatcher, do we need to invoke
    a cleanup method on the listener?

Things I don't like:

  - CallContext - we already abuse the term "context" enough

  - There's something about using a context manager for prepare() that
    I like:

        with client.prepare(version='2.6') as cctxt:
            cctxt.call('select_host',
                       request_spec=request_spec,
                       filter_properties=filter_properties)

    but it seems a bit nonsensical

  - "endpoints" - better than api_objs, callbacks, proxyobj, etc.

  - we probably won't use BlockingRPCDispatcher anywhere, but I think
    it does a good job of showing the basic job of a dispatcher
    implementation