mod_bookmarks2: Add new module. This is the result of hacking during the Stockholm XMPP Sprint, for compatibility with older clients only doing Private XML XEP-0048. This module shouldn’t be loaded at the same time as mod_bookmarks, as both implement Private XML to achieve their conversion.

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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.