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: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.