Support per-msg MSGID

[gdw2]

The way I see it, a tag is specified in the Dial 'constructor' and used for both the tag and msgid fields for RFC3164 and RFC5424 msg formats respectively. This is fine for RFC3164 because the tag doesn't typically change from message to message. But for RFC5424, the msgid should likely change, based on the msg type.

RFC5424 detailing MSGID: https://tools.ietf.org/html/rfc5424#section-6.2.7

I'm looking for consensus that this should be considered a 'defect', then I can put my thinking cap on regarding solutions.