Use the package go/ast/astutil for adding CoreDNS to caddy and removing
the http server stuff from it as well - we're only a DNS server, no need
to caddy all the HTTP stuff as well.
Results in smaller binary and plugin_generate.go being much smaller as
well.
* Make CoreDNS a server type plugin for Caddy
Remove code we don't need and port all middleware over. Fix all tests
and rework the documentation.
Also make `go generate` build a caddy binary which we then copy into
our directory. This means `go build`-builds remain working as-is.
And new etc instances in each etcd test for better isolation.
Fix more tests and rework test.Server with the newer support Caddy offers.
Fix Makefile to support new mode of operation.