mod_pubsub_post: Allow specifying item id by a special mapping If `{http://jabber.org/protocol/pubsub}id` exists as an attribute at the top level then it is used, otherwise the previous behavior (.id if string)

This module blocks configured queries that change server state.

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

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