Implement "flush"

[AlexeyRaga]

There is this api in librdkafka:

/**
 * @brief Wait until all outstanding produce requests, et.al, are completed.
 *        This should typically be done prior to destroying a producer instance
 *        to make sure all queued and in-flight produce requests are completed
 *        before terminating.
 *
 * @remark This function will call rd_kafka_poll() and thus trigger callbacks.
 *
 * @returns RD_KAFKA_RESP_ERR__TIMED_OUT if \p timeout_ms was reached before all
 *          outstanding requests were completed, else RD_KAFKA_RESP_ERR_NO_ERROR
 */
RD_EXPORT
rd_kafka_resp_err_t rd_kafka_flush (rd_kafka_t *rk, int timeout_ms);

It could be useful in flushProducer, closeProducer and perhaps closeConsumer