Snap sync to not include pre-merge blocks and receipts

[jframe]

Description

As part of supporting history expiry where clients are no longer required to serve blocks and receipts pre-merge we might not be able to reliably download blocks and receipts before the merge block.

Modify the current snap sync mode to not download the bodies and receipts before the merge block.

Acceptance Criteria

• Snap sync mode downloads all headers from head to genesis
• Snap sync does not download bodies and receipts before the pre-merge block
• Has a configurable checkpoint above which we download blocks and receipts
• By default, the checkpoint is the merge block
• This becomes the new default for snap sync and checkpoint is replaced by this with the configurable checkpoint