Pandomain node key rotation reviews

@bryophyta has been working [key rotation support](https://github.com/guardian/pan-domain-node/issues/28) and would like the W&C team to review some preparator PR's. See:
https://chat.google.com/room/AAAAwWNibJM/HWU7vg8Ofho/krDXf6hJBAg?cls=10

[Replace iniparser dependency with better-maintained ini package](https://github.com/guardian/pan-domain-node/pull/75)
[Convert verification to async/await](https://github.com/guardian/pan-domain-node/pull/76)
[Add refresh behaviour tests](https://github.com/guardian/pan-domain-node/pull/77)
[Validate user before checking for grace period](https://github.com/guardian/pan-domain-node/pull/78)
[Change instantiation to use async builder method](https://github.com/guardian/pan-domain-node/pull/79)

Start with key rotation support in issue #28 and review preparatory PRs #75–#79, using the linked descriptions and the W&C team discussion for context. Done means providing the requested review and feedback on the dependency, async conversion, refresh tests, validation, and builder changes.

Written by the indexing model from the issue text.