[ZIP 403] Describe what is verified (for each combination of options) in zcashd

[daira]

https://zips.z.cash/zip-0403

[str4d]

As of zcashd 4.0.0, there is a -checkpoints flag (inherited from upstream) that is set to true by default. When enabled (i.e. by default), ConnectBlock has the following behaviour:

• Sprout JoinSplit proofs prior to the last checkpoint are skipped.
• Transparent ECDSA signatures prior to the last checkpoint are skipped.

Everything else is checked unconditionally. In particular, the following expensive checks are always performed:

• Sprout joinSplitSig.
• Sapling Spend and Output proofs.
• Sapling spendAuthSigs.
• Sapling bindingSig.

On a performance note, as of 4.0.0, transparent ECDSA signatures are verified in a threadpool, while all other expensive checks are performed serially. No batching is performed for any checks.