Updates to malicious validation batcher

[andyleiserson]

Some of these changes are general improvements, and some will be needed to use the batcher for DZKPs (currently the DZKP validator inlines an early version of the batcher from before it was refactored into a separate file -- this will be addressed in a later change).

• Make total_records optional in the batcher. This will be used for the unbatched DZKPValidator::validate API (which still uses the batcher to store intermediates, because that is easier than conditionally using different storage).
• Handle total_records correctly in the multiple batch case.
• Remove an unnecessary atomic for the pending record counter.
• Also track records waiting for validation in a bitvec, as a sanity-check.
• Invoke the batch validation function inside validate_record, to reduce duplication between the MAC and DZKP implementations.
• Report validation success/failure to parallel validate_record waiters.
• Add tests for the batcher.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.56098% with 8 lines in your changes missing coverage. Please review.

Project coverage is 92.77%. Comparing base (77ac0b0) to head (200604c). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ipa-core/src/protocol/context/batcher.rs	97.48%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1234 +/- ## ========================================== + Coverage 92.73% 92.77% +0.04% ========================================== Files 200 200 Lines 31486 31764 +278 ========================================== + Hits 29197 29470 +273 - Misses 2289 2294 +5 ```

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