Points to reconcile between MAC and DZKP malicious APIs

[andyleiserson]

• [x] Whether or not an atomic integer is used to count pending records in a batch
• [ ] MAC implementation stores offset within each batch, which means it doesn't need the offset when validate_record returns a batch to be validated. Possibly DZKP can do this as well?
• [x] MAC batcher implementation is still using Notify, doesn't have the stuff to communicate Error::ParallelDZKPValidationFailure.
• [ ] Code duplication between DZKP and MAC validators - the logic to cancel pending futures when validation fails should be unified
• [ ] Test that API can interop with seq_join (try_join). I noticed that test runs that include more than one record id hang, if one validate fails. try_join_all works, ctx.try_join does not