Fix gas accounting for seal proof verification

[anorth]

Upcoming changes to built-in actors to implement direct data onboarding will invoke the batch_verify_seals syscall directly from the miner actor. In the past, this syscall has been invoked only from the power actor's cron handler. It does not charge the correct amount of gas, though there's no apparent reason why it can't. This syscall should instead charge the correct amount of gas. Doing so won't have any effect on the existing use since the cron originator has unlimited gas budget.

Fixing this is necessary for the deployment of direct data onboarding. I don't think it needs a FIP since the end result is just that the syscall works as expected, with no change to existing on-chain observable behaviour. Any necessary things can be specified in the direct onboarding FIP.

• Fix the comment in kernel/mod.rs by basically deleting it. The call is not privileged and should charge gas (reference to the pre-payment is both wrong and irrelevant here).
• Delete the comment in kernel/default.rs
• Fix the gas calculation in kernel/default.rs. A a minimum this might involve changing the price list item for verify_seal_base from 2000 to a real number for per-proof verification.

Changing the verify_seal_base price to 34721049 the amount hardcoded in the power actor, would then price this the same as the current cost of seal proof validation. However this price might (?) include an empirical discount that assumes some unknown number of additional proofs are submitted in the same epoch which can be verified in parallel. But a fixed price per item also obviously doesn't include a discount for the known number of proofs that actually are submitted when this is called directly from the miner.

FYI @ZenGround0 @Kubuxu

[Stebalien]

Couple of things to note:

1. The current gas price assumes 8-way parallelism (I think?), as far as I know. The gas price will have to be tiered (i.e., charge per 8 proofs, not per proof).
2. There may be some per-invocation overhead that's not being considered. This syscall effectively spins up a worker pool to process proofs in parallel.
3. We'll need to very carefully audit this function.

[Kubuxu]

There are two things in the current price, these is empirical discount from parallelism, on the other hand there were some improvements to proof verification performance since then.

There is also the debate whether the parallelism should be discounted because unoccupied threads are useful for processing alternative tipsets.

[arajasek]

I've benchmarked proof verification calls using code in #1882.

I propose doing away with any parallelism discounts, and simply charging for the cost of verifying the proofs. I propose we continue to verify them in parallel (assuming multiple proofs), but charging just based on the number of proofs.

Here are the results in nanoseconds:

For category 32 GiB (based on 48745 results): 
  - Average: 4048979.97 
  - Median: 4080682.00 
  - 60th Percentile: 4178855.00
For category 64 Gib (based on 30431 results): 
  - Average: 3927112.38 
  - Median: 3967278.00 
  - 60th Percentile: 4062714.00

These are obtained after stripping out all failures (bad proofs), and stripping out the top and bottom 2% of results. Neither of those operations changed the result significantly.

I propose using the 60th percentile value of the 32 GiB sector for all sector sizes, which would yield a cost of 41.78M gas per proof verification. This is only slightly more than the current cost of 34.72M gas per proof.

[Stebalien]

This seems like a reasonable approach. If it becomes an issue, we can likely change it later. The core issue here is that requiring additional cores has an opportunity cost.