va: use only half the available timeout on primary validation

If we assume a primary validation and the MPIC validations take approximately the same amount of time, and the overall timeout for an RA->VA request is 90s, then a request that takes 50s for primary validation will fail secondary validation because not enough time is left.

We can reflect this more accurately by having the VA look at the remaining time on its context object, cut that time in half, and putting that number as a timeout for primary validation specifically. This can allow us to drop failing requests a little earlier. This can also allow us to confidently put a shorter timeout on the VA->RVA connection.

Start by locating the VA code that performs primary validation and reads the request context's remaining time. Trace how the timeout is passed to primary validation and how the VA-to-RVA request is timed. Done means primary validation uses half the remaining context time and the shorter downstream timeout can be applied without breaking secondary validation.