Attempt to ping the ZeroMQ plugin hub to verify connection is alive.

        If ping is successful, record timestamp.
        If ping is unsuccessful, call `on_heartbeat_error` method.

def ping_hub(self):
        '''
        Attempt to ping the ZeroMQ plugin hub to verify connection is alive.

        If ping is successful, record timestamp.
        If ping is unsuccessful, call `on_heartbeat_error` method.
        '''
        if self.plugin is not None:
            try:
                self.plugin.execute(self.plugin.hub_name, 'ping', timeout_s=1,
                                    silent=True)
            except IOError:
                self.on_heartbeat_error()
            else:
                self.heartbeat_alive_timestamp = datetime.now()
                logger.debug('Hub connection alive as of %s',
                             self.heartbeat_alive_timestamp)
                return True