mod_http_muc_log: Use XEP-0421 ID over nickname for XEP-0308 This prevents someone else from joining with the same nickname as someone who left, and "correcting" their messages.

This module blocks configured queries that change server state.

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

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