Update devp2p chain to pos

[mpaulucci]

This PR has two changes:

• Instead of using the Geth chain, let's decouple them and generate our own inside Hive repo.
• Updates the chain to pos. This will allow only-pos clients (see ethrex) to be able to not have to mantain a hive fork. Also, starting on eth/69, we don't even serve pre merge blocks. The chain generation script has been added to simulators/devp2p/mkchain.sh

The the Geth one for reference: https://github.com/ethereum/go-ethereum/blob/master/cmd/devp2p/internal/ethtest/mkchain.sh

[fjl]

I'm updating it upstream in go-ethereum instead. We want to use the same chain for snap tests, so it makes sense to update it for both.

[mpaulucci]

Thanks!