Consider adding a "message_tag" to Message

[edmonds]

Discussion here: http://lists.redbarn.org/pipermail/dnstap/2015-March/000020.html.

[edmonds]

Also here: http://lists.redbarn.org/pipermail/dnstap/2015-November/000027.html.

[pavel-odintsov]

Hello!

Thanks for discussion, I relieved it in details. Special flag for caching information is also very good idea. We are on they way to deploy DNS TAP for our network and I think we could find other required fields ;)