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.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```