* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
* Remove debug from interface and methods
* remove debug queries from etcd
* remove debug queries from k8s - they were not used
* And remove from mw/proxy-google as well
* Remove debug query test
Add a Reverse method to BackendService because different backends want
to to do diff. things. This allows etc/k8s to share even more code and
we can unify the PTR handling.
* Add ServiceBackend interface
This adds a ServiceBackend interface that is shared between etcd/etcd3
(later) and kubernetes, leading to a massive reduction in code. When
returning the specific records from their backend.
Fixes#273
