Add resolver methods for SOA and NS records

[rotty]

These are useful if you want to use the resolver to retrieve:

• The domain master address, to know where to send DNS UPDATE
  messages to.
• The authoritative nameservers for a domain, to monitor them for the
  propagation of an update.

---

@blujekyll: I know you are busy on PR #849, so don't feel obliged to
handle my PRs; I can rebase them when #849 has landed.

[bluejekyll]

thanks, I should be merging that soon!

[codecov]

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[bluejekyll]

This looks good, though this raises an a question in my mind.

What should our general practice be for the less often used record types? There is a generic lookup method, I'm just trying to figure out the best practice here.

[rotty]

I'm not sure where to draw the line either, it seems to be overkill to include methods for all record types. My gut feeling is that (glancing over https://en.wikipedia.org/wiki/List_of_DNS_record_types), the most common record types are covered now.

Some additional use-cases that seem plausible: When someone implements an SSH client, SSHFP lookup would be relevant, an email application may want OPENPGPKEY lookups and TLS clients may want to do TLSA lookups. I have no idea how commonly these are actually deployed.

[bluejekyll]

But for the less common types, they could use the generic lookup method, right? If we really want a function per lookup type, we could just expand the macro to be all types and be done with it... just seems like overkill.

[rotty]

Sure, the generic lookup would have sufficed for my purposes as well, it just seemed like NS and SOA lookups are one the same level of "commonality" as MX and SRV.