Rename `tag` to `match_value`

[mirceaulinic]

Given that many messages won't have a unique identification tag (e.g., something like ROUTING-BGP-5-MAXPFX or at least bgp_read_message) we'll need to find other ways to match the message. In that case it might be confusing to use tag where there's not. But that's just a detail, looking for more feedback.