This middleware allows you to round robin a/aaaa records in a reply and
maybe more in the future (i.e.) sort a packet?

Fix early stage warning

I'm guessing this was the intended meaning. =)

Remove things not supported, fix docs.

When we cannot bind to the port, return an error which will make
CoreDNS fail startup. Still a bit hackish, but good enough.

Fix upstream tests in proxy middleware

Errors directive testing and fixing

Drop a few tests and make it work and compile.
Also add the documentation: errors.md

Fix middleware log

Context

Rename the old Context to State and use context.Context in the
middleware for intra-middleware communication and more.