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.

This module produces more detailed HTTP logs for Prosodys built-in HTTP
server. The format is similar to that of Apache and go into Prosodys
normal logs at the `info` level.