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.

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.