Return a thread local :class:`dossier.label.LabelStore` client.

def label_store(self):
        '''Return a thread local :class:`dossier.label.LabelStore` client.'''
        if self._label_store is None:
            config = global_config('dossier.label')
            if 'kvlayer' in config:
                kvl = kvlayer.client(config=config['kvlayer'])
                self._label_store = LabelStore(kvl)
            else:
                self._label_store = self.create(LabelStore, config=config)
        return self._label_store