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_last_transaction.py

@@ -52,4 +52,4 @@ else:
     sock.send(bytes("QUERY-LAST-TRANSACTION".encode()))
 
 response = sock.recv(1024)
-print(response.decode('ascii'))
+print(response.decode('utf_8'))