make our zone parser adhere to RFC2308 wrt implicit TTLs and add test

[ahupowerdns]

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[ahupowerdns]

#3673 references this issue. Note that we need to check if we talk about this in the documentation and SHOUT about this in the release notes

[Habbie]

Code and test look good! For release notes, we should just handle that in this PR already so we don't forget.

[rgacogne]

pySNMP-related failure in Travis, rebasing should fix that!