Implement better DNS Retry logic (Agency 2 Pilot: EXO 2.4; DMARC Failures)

[schrolla]

Agency 2 manages their DMARC policy outside of EXO but could still verify they meet the policy via DNS lookups. We should investigate why they failed.

[gdasher]

The DMARC policy in the rego is based purely on DNS. Looking at Agency 2 and Agency 1's data, the DNS results are not being correctly populated for a subset of their domains, perhaps due to transient errors.

I believe the appropriate fix for this issue is better retry logic for DMARC. I am retitling the bug as soon and unassigning from me for prioritization of implementation in Dolphin (or deferral to later release).

[gdasher]

Specifically, on transient DNS failures, we should retry as well as differentiate this error condition from NXDOMAIN. Logic should cover all policies reliant on DNS data (DMARC, SPF, DKIM).

[gdasher]

Closing as duplicate of #41