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 lets you block connections to remote servers at the s2s
level.

``` {.lua}
s2s_blacklist = {
    "proxy.eu.jabber.org",
}
```