mod_pubsub_serverinfo: Update to use mod_server_info (fixes #1841)
...but only for Prosody trunk users, because 0.12 mod_server_contact_info does
not use the new API in mod_server_info.
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summary: Log a message once per minute
...
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.