Anonymize IP logging

[masonicboom]

I made a module called ipscrub that does this: http://www.ipscrub.org. It
hashes the IP address with an ephemeral salt, so that you can match up
requests from the same IP (using the hash), but each time the salt cycles,
it becomes impossible to match an IP address with a hash in the logs.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,226003,279264#msg-279264