Add a "cache_status" enum to Message

[edmonds]

Semantics described in http://lists.redbarn.org/pipermail/dnstap/2015-February/000017.html.

[edmonds]

Alternate proposal described here: http://lists.redbarn.org/pipermail/dnstap/2015-September/000024.html.

[jedisct1]

+1 for the alternate proposal 👍

[chantra]

adding https://www.mail-archive.com/dnstap@lists.redbarn.org/msg00041.html to this.