This checks if the next middleware to be called is nil, and if so returns
ServerFailure and an error. This makes the next calling more robust and
saves some lines of code.
Also prefix the error with the name of the middleware to aid in
debugging.
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
