Move all (almost all) Go files in middleware into their
own packages. This makes for better naming and discoverability.
Lot of changes elsewhere to make this change.
The middleware.State was renamed to request.Request which is better,
but still does not cover all use-cases. It was also moved out middleware
because it is used by `dnsserver` as well.
A pkg/dnsutil packages was added for shared, handy, dns util functions.
All normalize functions are now put in normalize.go
For prometheus use the plain value, not a pointer and change all usages.
Allow AXFR to be requested over udp as well and some other more log
printed when commencing an AXFR.
Allow specifying a primary server and retrieve the zone's content.
Add tests and an Expired bool to zone struct, to stop server zones
that are expired. The zone is retrieved on Startup, no updates of
changed content are done. We also don't respond to notifies yet.
