Make Eth1 distance runtime configurable

[protolambda]

Not sure if I'm doing this right, but needed this for a new lighthouse/nimbus dev testnet. Will test it now for genesis and report back.

[protolambda]

Genesis worked 🎉
Testnet details here: https://github.com/protolambda/fafafa

Need to fix the constant-number tests though, but not sure how that works in nim. Can someone help out here?

[zah]

Thanks @protolambda, the patch is almost perfect. I've pushed only a small change that should fix the CI issues.

[protolambda]

Thanks zah, appreciate it. But feel free to make any changes, I don't write nim enough to see what works best here, or how it should work in nimbus.