va: Handle IPv4 fallback for HTTP(S) redirects

When performing an HTTP-01 validation, the VA will try to fall back from IPv6 to IPv4 if the connection fails; and it will follow HTTP(S) redirects. However, the current structure of processHTTPValidation cannot handle a case where both are needed.

Reproducer: Publish IPv6 & IPv4 addresses in DNS for a test host. Misconfigure networking on the test host so that IPv6 connections will fail. Configure a self-signed certificate for HTTPS. Configure the Web server to redirect all HTTP traffic (including ACME challenges) to HTTPS. Attempt HTTP-01 validation (e.g. certbot certonly --webroot). It will fail: Boulder will retry to IPv4, and follow the redirect - but when trying to validate the redirect target, it will only try IPv6 and not attempt to retry to IPv4.

Start at processHTTPValidation and reproduce the case with both IPv6 and IPv4 DNS records, failed IPv6 connectivity, and an HTTP-to-HTTPS redirect using a self-signed certificate. Done means HTTP-01 validation follows the redirect and still retries the redirect target over IPv4 when its IPv6 connection fails.