net/nmsg: Upgrade to 1.2.0

nmsg (1.2.0)

• Add support for Kafka i/o in JSON and binary forms.
• Add support for prometheus metrics export.
• Replace mutex protected counters and flags with stdatomic operations.
• New command line options: --kafkakey, --readkafka, --writekafka, --promport
• New functions: nmsg_input_open_kafka_endpoint(), nmsg_input_open_kafka_json(), nmsg_output_open_kafka_endpoint(), nmsg_output_open_kafka_json()

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