'Validates that supplied request token is registered and valid.
Note that if the dummy request_token is supplied it should validate in
the same nearly the same amount of time as a valid one.
Bad:
if request_token == self.dummy_request_token:
return False
else:
return storage.has_request_token(request_token)
Good:
return (storage.has_request_token(request_token) and
request_token != self.dummy_request_token)'

def validate_request_token(self, client_key, request_token):

    raise NotImplementedError('Subclasses    must    implement    this    function.')
