Improve UTF-8 handling in avrorouter

The json_stringn function should be used instead of the json_string to
allow null characters as well as non-null terminated strings to be
embedded in the JSON values.

The CDC example Python programs now decode the data as UTF-8 instead of
ASCII.

server/modules/protocol/examples/cdc_kafka_producer.py

@@ -45,7 +45,7 @@ while True:
 
       while True:
          rbuf = rbuf.lstrip()
-         data = decoder.raw_decode(rbuf.decode('ascii'))
+         data = decoder.raw_decode(rbuf.decode('utf_8'))
          rbuf = rbuf[data[1]:]
          producer.send(topic=opts.kafka_topic, value=json.dumps(data[0]).encode())
          producer.flush()