refactor cdxserver, add tests focused on wsgi_cdxserver, add docstrings.
  align cdxops function interfaces - all cdx_iter.
  move module functions / common ops to class methods
  support both 0/1 and true/false for boolean parameters
  move CDXObject to text conversion to wsgi_cdxserver (may have broken
    embedded cdxserver mode).
  pass config object as function arg rather than as global var.