Ability to block producer in case the underlying queue is full.

Author: Hana Dusíková

include/cppkafka/producer.h

@@ -86,9 +86,10 @@ class CPPKAFKA_API Producer : public KafkaHandleBase {
      * The policy to use for the payload. The default policy is COPY_PAYLOAD
      */
     enum class PayloadPolicy {
-        PASSTHROUGH_PAYLOAD = 0,            ///< Rdkafka will not copy nor free the payload.
-        COPY_PAYLOAD = RD_KAFKA_MSG_F_COPY, ///< Means RD_KAFKA_MSG_F_COPY
-        FREE_PAYLOAD = RD_KAFKA_MSG_F_FREE  ///< Means RD_KAFKA_MSG_F_FREE
+        PASSTHROUGH_PAYLOAD = 0,                   ///< Rdkafka will not copy nor free the payload.
+        COPY_PAYLOAD = RD_KAFKA_MSG_F_COPY,        ///< Means RD_KAFKA_MSG_F_COPY
+        FREE_PAYLOAD = RD_KAFKA_MSG_F_FREE,        ///< Means RD_KAFKA_MSG_F_FREE
+        BLOCK_ON_FULL_QUEUE = RD_KAFKA_MSG_F_BLOCK ///< Producer will block if the underlying queue is full
     };
 
     /**