command:serve: add SilentRequestHandler to avoid double-logging requests.

command:serve: add SilentRequestHandler to avoid double-logging requests.

They were being logged by the request handler, and then logged again
by WSGI_Object.log_request.  After this patch, requests are only
logged when logger.level is at a <= level (e.g. closer to DEBUG)
than the WSGI_Object.log_level (usually INFO).  For example, in
Serve_setup_logging,

  self.logger.setLevel(logging.INFO)

will get request logs printed, but

  self.logger.setLevel(logging.WARNING)

will not.