refactored MultiplexManager

@digitalkaoz i should be writing slides, but ended up pondering your changes a bit more. here are some refactorings:

1) removed request property
2) move some code into the ``sanitizeRequest()`` method
3) ensure that subrequests do not need to call ``sanitizeRequest()``
4) introduced a ``MultiplexExternalManager`` that provides the external support, so that the Buzz dependency can become optional