mod_muc_rtbl: move use of "private" attributes to single function This way, we reduce the scope where you can have a typo in the attribute name to that function and we encourage users to actually always call update_hashes.

This module blocks configured queries that change server state.

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

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