'Validates that supplied client key is a registered and valid client.
Note that if the dummy client is supplied it should validate in same
or nearly the same amount of time as a valid one.
Bad:
if client_key == self.dummy_client:
return False
else:
return storage.has_client(client_key)
Good:
return storage.has_client(client_key) and client_key != self.dummy_client'

def validate_client_key(self, client_key):

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