mod_rest: Move most of XEP-0432 handling into JSON mapping schema The pre- and post-processing is still needed to encode/decode the JSON since util.datamapper can't (currently) do this.

This module blocks configured queries that change server state.

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

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