Update reward calculations to v0.11.0 - Handle u64 overflow

[ghost]

Issue Addressed

#920

Proposed Changes

• Divides by EFFECTIVE_BALANCE_INCREMENT both dividend and divisor on reward calculations

• Suffix _ebi on each effected variable (To avoid any future confusion)

Additional Info

• Reference handle rewards overflow  ethereum/consensus-specs#1635

[paulhauner]

Thanks @hermanjunge!

@michaelsproul perhaps you'd like to check over this one as well?

This PR would put us in a middle-ground between spec v0.10.x and v0.11.x, but that's probably not too bad considering we don't expect to see this scenario on testnets. We're going to need to get to v0.11.x before we get to multi-client testnet anyway since that's where the networking spec is functional.

[paulhauner]

Also, @hermanjunge it would be handy for future PRs if you could enable Github Actions on your end then our CI will run :)

[ghost]

@paulhauner Done. Will squash both commits!

[ghost]

Jobs completed. I will avoid making a new PR with this other branch. Too much noise. https://github.com/hermanjunge/lighthouse/actions/runs/57159324

Next time PR will be from a branch different than master.

[michaelsproul]

This looks good to me. It doesn't break any functionality of v0.10.x that was worth keeping, so I'm fine to merge and exist "between specs" for a bit.

[paulhauner]

I've triggered CI on our end :)