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.

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.