Bugfix: sync crossing merge boundary failing pre-merge header validation rules

[garyschulte]

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Description

#4190 introduced a regression where a batch of blocks that cross TTD can fail to validate. For example, if 200 blocks are fetched to import and 100 of them are pre-merge, and 100 post-merge, the pre-merge block validator will get selected because we have not yet imported a block that crosses the terminal total difficulty.

Acceptance Criteria

• fast sync block imports should use the correct/corresponding block header validators when importing batches, even if a batch crosses the terminal total difficulty block

Steps to Reproduce (Bug)

1. fast sync ropsten using the merge commit of #4190

Expected behavior: [What you expect to happen] all fast syncs complete

Actual behavior: [What actually happens] all fast sync methods fail to import post-merge blocks

Frequency: [What percentage of the time does it occur?] 99.5% of the time (depending on the batch size and the roll of the dice where the block range boundary is)