Respect TTL

[bookmoons]

Adds expiration support to the cache. Sets up RRs to expire based on TTL.

If the patch is accepted I intend to make a submission to k6 using dnsr as the caching resolver, discussed in grafana/k6#726.

The GooglePTR test seems to be timing out on my system but the expiration tests pass.

Closes #31.

[bookmoons]

Cache get() ended up with some nesting. Went for minimal call overhead but it does read awkward. If you think it's too messy I can factor it.

When expiration is enabled, TTL is stored in the RR. The only use is in stringification to zone file format. If that's not worth storing it I can take it out. The current code is stringifying with a static TTL of 3600.

I'm not familiar with the godoc system. Should I be doing anything to make those docs update?

[ydnar]

Thanks for the PR! Only a few minor changes and this should be good to go.

[bookmoons]

Thanks a lot for this guys. Will go over all of it.

[ydnar]

Thanks!

[bookmoons]

Thanks for the review and merge guys.