WIP: Avoid re-reading CVRs during 'Tabulate' step of validation modal

[yezr]

As described by @artoonie in this comment on #679, we currently read the CVRs twice: once to get the total number of ballots for the validation modal and again during the Tabulation. Let's think about how to avoid doing that.

Some things to think about....

• Multi-winner IRV currently reads the CVRs in for each round, excluding the winner of the previous "contest." This will need to be addressed.
• We should use LoadedCvrMetadata to store hashes of the files and confirm during runtime that the CVRs haven't changed since they were used to get the ballot count. If they have been changed, we should require re-running the ballot count.

[HEdingfield]

Discussed a bit in this PR comment thread.