core: attempt at alternative tx-level prefetcher

[holiman]

This is a proof-of-concept test of an idea based off a discussion with @rjl493456442 . We can try to prefetch transcations in the current block.

Current, prior to this PR, we have

1. Prefetcher which executes the next block opportunistically. This is not used in regular operations, since we usually don't have the next block -- only while catching up.
2. Prefetching the trie. This happens post-execution: after executing tx 1, we try to prefetch the trie so we later on can do the commit faster.

This PR changes the 1. prefetcher, so that instead of pre-executing the next block, we skip ahead across a third of the transactions in the block, and (opportunistically) execute it in order to pre-load stuff into caches.

[holiman]

I've had this running for a couple of days now, versus master. I see no meaningful difference with this PR, so closing.