mod_storage_xmlarchive: Advertise capabilities (none atm) Both 'total' and 'quota' would be prohibitively expensive for the kind of archives this module targets as it would require counting items across the entire archive.

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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.