Rel 0.9.16 version bump & CHANGELOG update

Base support for managing root NS records

Co-authored-by: Viranch Mehta <email@viranch.me>

This will result in less churn in tests for root NS support and allow us
to enable/disable things easily as needed.

Fix handling of null MX records and add test for behavior

add Scaleway provider

This reverts commit f1fd63205e.

Rework _AggregateTarget to dynamically handle SUPPORTS

Rel 0.9.15 version bump & CHANGELOG update

* Zone object no longer treats them special, some tests needed adjusting
  b/c of this, some provider's tests may also need adjusting, though
  they should not plan changes since they won't (yet) have
  SUPPORTS_ROOT_NS
* _process_desired_zone filters and warns when not supported
* YamlProvider supports them
* TinyDnsBaseSource supports them

Update readme post extraction

Validate record fields that should hold FQDNs w/tests

Test that CNAME does not allow url and fqdn w/path