mod_rest: Fix nested message stanzas in XEP-0297 containers It's all a bit awkward because it doesn't normally use the whole schema directly, but rather checks which stanza type is used and picks out that property as a schema. Which also prevents references from working.

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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.