Eliminate some obviously useless threading

[alexpyattaev]

Problem

pub fn ed25519_verify_disabled(batches: &mut [PacketBatch]) in perf/src/sigverify.rs
does pretty much no work (sets a boolean flag for each Packet in PacketBatch) and yet it creates doubly nested parallel iterators to achieve this results. It further does this on rayon's global thread pool so it is impossible to do accounting on this activity.

Summary of Changes

Changed the function to set the flags in single thread. Unless there are several thousands of packets in those batches, this will be faster.

[steviez]

Hi @alexpyattaev - The PR description and code changes don't match; the problem description refers to sigverify but the actual diff is touching rocksdb.

Ignoring that and responding to the rocksdb changes, I still think wiring things up to be configurable (on the CLI for now since that is where everything else is) is the right move for now to allow some customization & leave an escape hatch for later.

I have #4214 which does that; I opened this a couple days ago but didn't add reviewers and/or draw attention to it in light of the holidays. I think our efforts were mostly tangential here, but we can make sure to coordinate better in Slack to avoid any duplicated efforts moving forwards

[alexpyattaev]

Sorry I have pushed the wrong diff. Should not make PRs late at night. Especially the "small and trivial" ones.