mod_pubsub_mqtt: Support atom_title payload type
This commit adds the ability to publish and subscribe with arbitrary payload
types. It has a breaking change, which is that topics are now of the form:
HOST/TYPE/NODE
Currently supported types are utf8_data, json and atom_title.
16 Your connection is encrypted using the %s protocol, which has been demonstrated to be insecure and will be disabled soon. Please upgrade your client.