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.

Dyn will still match them, even while they're empty before we've had a
chance to add the respons pools to them which is BAD and caused a
medium-size outage (thankfully not as bad as it could have been.) Ideally
we'd use publish=False and stage things, but that seems broken in the client
lib, there's no way to send publish=N. I also tried sending the
response_pool_ids as part of the create calls and response pool config if
one didn't exist, but neither of those routes worked :-(

* spelling: ancillary
* spelling: antarctica
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This change hit a line length limitation, so I'm wrapping it and
adding a period which appears to match local style...

* spelling: quicker
* spelling: response
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* spelling: useful
* spelling: separately
* spelling: zone