mod_pubsub_feeds: Use ETag instead of problematic If-Modified-Since Because it set If-Modified-Since to the last poll time instead of the exact value of Last-Modified in the last response, it would not work with some HTTP servers.

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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.