Returns a specific engine.

def get_engine(self, app=None, bind=None):
        """Returns a specific engine."""

        app = self.get_app(app)
        state = get_state(app)

        with self._engine_lock:
            connector = state.connectors.get(bind)

            if connector is None:
                connector = self.make_connector(app, bind)
                state.connectors[bind] = connector

            return connector.get_engine()