approval-voting: experiment with batching signature checks

[sandreim]

We could additionally gain some performance by batching signature checks. Right now we check them individually by handling CheckAndImportAssignment and CheckAndImportApproval. We have to test if there are any significant gains by doing something like, buffering signature checks until reach a limit or N milliseconds have passed since we started buffering. This would play nicely with #731.

Another approach is to do the batching in approval-distribution but I am not really sure if this would be more effective.

[burdges]

I vaguely recalled "batching" here does not mean "cryptographic batching" like I mentioned in #730?

[sandreim]

Yep, it is actually not, but stacks with "cryptographic batching" as we scale up validator and parachain count. It's mostly about improving our architecture of processing approval voting gossip messages and slicing the CPU intensive work.

[burdges]

cc paritytech/polkadot#7554