DevP2P support for eth/70

[jflo]

Update this issue with a proposed design analysis for review and discussion before proceeding. It should include any opportunities to refactor or adopt IoC while implementing.

Implement EIP-7639

Disconnect peers who request blocks prior to the merge block.

Add user action (prefer subcommand, but a CLI arg of some sort would work too) to drop the blocks prior to the merge as specified in EIP.

Bonus: Log to the user how much space was recovered