Consider a depressolver implemented against the interface

[kuritka]

I've been considering implementing a depressolver against the interface for a long time - mainly because of the generated mocks, which would make testing the GSLB controller much easier. Practice says that you shouldn't change the implementation for testing, but it would be more than appropriate at this point.

if accept the new interface, implement the following steps:

• the implementation of the Resolver interface
• refactor
• modify the tests if necessary
• generate MOCK