Buffered producer thread safe (#72)

* Thread safe buffered producer

* Using single mutex version

* Changed based on feedback

* Changes based on latest review

* Added flush counter

include/cppkafka/producer.h

@@ -82,6 +82,7 @@ public:
      * 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
     };