Adds a new source requested in #239. This source allows a user to pull data from
a legacy system (Bind9, etc.) that does not have an API/existing provider via
AXFR Zone Transfer.

boto3's methods as fallback

...for testing or emergencies

Implements the unsupported records skipping fix [proposed by ross](https://github.com/github/octodns/issues/176#issuecomment-359294960) in #176 for DNSimple and additionally for NS1.

Fixes #176 and also the NS1 version of it (`AttributeError: 'Ns1Provider' object has no attribute '_data_for_DNSKEY'`) that currently affect domains that have DNSSEC enabled.

This will ensure unique refs for different zones. Without them the ref isn't
enough to make sure we're looking at the right thing (notably when we're
gc'ing old health checks.) This also adds a bit more debugging around health
checks.