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

def validate_access_token(self, client_key, access_token):

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