`persist` is a hotspot when processing blocks because it is run at least
once per transaction and loops over the entire account cache every time.
Here, we introduce an extra `dirty` map that keeps track of all accounts
that need checking during `persist` which fixes the immediate
inefficiency, though probably this could benefit from a more thorough
review - we also get rid of the unused clearCache flag - we start with
a fresh cache on every fresh vmState.
* avoid unnecessary code hash comparisons
* avoid unnecessary copies when iterating
* use EMPTY_CODE_HASH throughout for code hash comparison
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