va: wait for all remote results before returning

[jsha]

Right now, if two remote VAs return errors before the other remote VAs complete validation, the local VA will cancel the remaining RPCs and return early to the RA. This is an optimization that may save us a little work and speed up responses in some cases.

However, it adds some complexity to our VA code. As we add code to align with the MPIC ballot, we're adding further complexity. It would be nice to reduce some complexity at the same time.

The BRs say:

The CA SHALL record at least the following events:
Multi‐Perspective Issuance Corroboration attempts from each Network Perspective,
minimally recording the following information:
• a. an identifier that uniquely identifies the Network Perspective used;
• b. the attempted domain name and/or IP address; and
• c. the result of the attempt (e.g., “domain validation pass/fail”, “CAA
permission/prohibition”).

While we could record the result of the canceled attempts as "canceled", it would be nicer to simply be consistent and wait for all results.

Historical note: for a long time we had the feature flag MultiVAFullResults to allow choosing this behavior. We deprecated it in June because we weren't using it, and removed it entirely this week. That's okay!