Define doLookupProtocol in a common place so that it can be used across modules

[vishalmohanty]

Currently every module is having to define doLookupProtocol in order to get IP addresses of either CNAME records, MX records, NS records, etc.

I was able to define DoLookupProtocol in common.go but faced issue in running unit tests like alookup_test.go because the mocked DoMiekgLookup was not being called. Instead the actual implementation in miekg.go was being invoked.

There should be some way of injecting dependencies or mocking functions of other modules, which will help us abstract out doLookupProtocol. Investigate that.

[zakird]

It probably makes sense to define the function here as part of the core library. https://github.com/zmap/zdns/blob/master/modules/miekg/miekg.go