Fix flawed response logic for COOKIE-less queries

Fix flawed response logic for COOKIE-less queries

The "yield" keyword does not cause a function to return.  By design,
get_responses() may yield multiple DNS responses in a single call.  As
currently implemented, CookieHandler.get_responses() sends two responses
to each client query that does not contain a COOKIE option.  Make the
logic in that method consistent with code comments by only sending one
response to every query - either SERVFAIL or BADCOOKIE, never both.