Drop SPF support and replace it with TXT records

[istr]

Fixes #43. Prepare for octodns v2.

[istr]

My thinking was that they'd be deprecated in core with the message and then the type would be removed in 2.0 with the expectation that users would change things in their config.

The test part of this change can/should happen now, but the removal of the type would need to wait until after 2.x or otherwise users who happen to have an SPF defined would see it go unmanaged and not be able to change or delete it.

OK, fair enough. I would then suggest

• keep the reading part of it (so SPF is still supported when found remotely),
• drop the writing part, and add the ability to change SPF to TXT on the next change.

This way

• Users will see the change when checking and can decide if they want to downgrade for now.
• Users will see the change and have the rewrite managed automatically for them.

[ross]

• keep the reading part of it (so SPF is still supported when found remotely),
• drop the writing part, and add the ability to change SPF to TXT on the next change.

That actually might make sense to do in octoDNS core as of 2.0, allow people to still read them, but not create them.

• drop the writing part, and add the ability to change SPF to TXT on the next change.

I think this would potentially run into trouble if there were also TXT records defined at the same node. It'd need to merge the records or otherwise there'd be a duplicate/conflict. If we want to do anything like this it'd probably best happen in core so that it'd be universal across all providers. This would be another option for 2.0, in addition to/replacement of removing SPF write support.

I'd say that the providers can/should stay untouched until things are removed in 2.0 at which point there can be a pass of PRs to remove SPF support.