Support batch verification of JoinSplit Groth16 proofs

[conradoplg]

Motivation

For #322 we added JoinSplit on Groth16 verification, but without batch support. The reason for this is that zcash_proofs does not return the verification key in a format suitable for batching (we need a VerificationKey, and not a PreparedVerificationKey).

Add support to batch verification, probably by submitting a PR to zcash_proofs to make it return the JoinSplit Groth16 verification key as required. See discussion

Specifications

Designs

Related Work

Follow up to #322

[mpguerra]

Hey team! Please add your planning poker estimate with ZenHub @conradoplg @dconnolly @jvff @oxarbitrage @teor2345 @upbqdn

[conradoplg]

I believe this is is nice-to-have but not that important right now:

• I suspect most JoinSplits are pre-Canopy and thus don't need to be verified due to the checkpoint verifier
• The performance gain with the remaining JoinSplits won't be too great since there won't be too many of them.

I'm not 100% sure though. In any case it seems this could be moved out of the sprint if needed.

[teor2345]

• I suspect most JoinSplits are pre-Canopy and thus don't need to be verified due to the checkpoint verifier

If we want to skip even more JoinSplits, we can increase the default checkpoint height to just before NU5 activation. (So we don't have to worry about the testnet NU5 rollback.)

[conradoplg]

Tracked in #3153. Can be done if we detect it's a bottleneck.