Calculate max epoch and churn for slashings once

[rkapka]

What type of PR is this?

Optimization

What does this PR do? Why is it needed?

When processing a slashing, we need to know the max exit epoch and the validator churn. The function that returns these values is MaxExitEpochAndChurn, which internally calls ReadFromEveryValidator on the state, which is an expensive operation. Currently we call MaxExitEpochAndChurn for every proposer slashing and every validator index of an attester slashing. It can get very expensive especially in the case of an attester slashing with many validators being slashed. Since MaxExitEpochAndChurn only requires the state, it can be called just once before any slashing processing.

Here is an image showing that processing slashings was very CPU intensive on Holesky

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.