This prevents tracebacks during shutdown:

  Unhandled exception in thread started by <bound method Thread.__bootstrap of <Thread(Thread-1, stopped daemon 139820471113472)>>
  Traceback (most recent call last):
    File "soft/lib/python2.7/threading.py", line 774, in __bootstrap
      self.__bootstrap_inner()
    File "soft/lib/python2.7/threading.py", line 814, in __bootstrap_inner
      (self.name, _format_exc()))
    File "soft/lib/python2.7/traceback.py", line 242, in format_exc
      return ''.join(format_exception(etype, value, tb, limit))
    File "soft/lib/python2.7/traceback.py", line 141, in format_exception
      list = list + format_tb(tb, limit)
    File "soft/lib/python2.7/traceback.py", line 76, in format_tb
      return format_list(extract_tb(tb, limit))
    File "soft/lib/python2.7/traceback.py", line 100, in extract_tb
      linecache.checkcache(filename)
  TypeError: 'NoneType' object is not callable

Also rename the method because this is not longer *Z*server.

This waitress close() API is not public part of API, so we still ignore
the case where the server thread does not stop after 5 seconds. This
happens if some HTTP connections are still open at the end of test.